As furious irrigators prepare to descend on Parliament House in Canberra, unified by their desire to tear up the Murray-Darling Basin Plan, many others are questioning what doing so would achieve.

The $13 billion plan to remove water from farmers and return it to the environment has never been popular in southern New South Wales, which is home for the vast majority of the protesters.

They torched a draft version of the plan, and now, six years after it was formally enacted, they still want to burn it.

As the plan has progressed, it has removed 2,100 billion litres of water from farmers across the basin, with most of that water coming from New South Wales and Victoria.

The fury and desperation in these communities cannot be understated.

When farmers sold their water to the Commonwealth, it cut the productive capacity of the regions, farmers left and businesses shut, and with them went pupils in schools and members of sporting and civic groups.

In the last six months of 2018, inflows into the lower Darling and lower Murray were in the lowest 9 per cent ever recorded. ( ABC News: Will Kendrew )

At the same time, water that was once used in these areas to grow rice, pastures for dairy and cereal crops is now traded downstream to produce almonds, citrus and grapes.

Meeting increased horticultural and environmental demand for water downstream puts parts of the river under intense stress and has led to the degradation of river banks and a huge transfer of wealth from one region to another.

But removing the basin plan will not unwind the damage these communities have suffered, nor will it return water that's been brought back.

There is a drought

The Murray-Darling Basin is in the grip of the worst two- to three-year drought on record.

The basin plan has no bearing on the immediate anger over the lack of water allocation, because the rules for sharing water are laid out in the Murray-Darling Basin Agreement, which hasn't changed substantially since the 1980s.

In the southern basin, New South Wales, Victoria and South Australia share the water contained in the major storages at the top of the Murray River (Hume and Dartmouth dams), Lake Victoria and the Menindee Lakes.

Winter rainfall over southern Australia has been declining over the past 20 years. ( Supplied: CSIRO/BOM )

South Australia's share of Murray River water is fixed at 1,850 billion litres a year under the agreement — not the basin plan.

New South Wales and Victoria must supply that volume equally from their shares of the Murray River before they allocate water to meet their own urban, industrial and irrigation commitments.

Rural news in your inbox? Subscribe for the national headlines of the day.

For the past few years, drought has meant tributary inflows to the Murray River have been miniscule, meaning the lion's share of water for South Australia has come from Hume and Dartmouth dams.

Victoria and New South Wales use differing methodologies for allocating water, with Victoria's water budget focusing on ensuring there are reserves for the following season; nevertheless, Victoria, on average, contributes double the amount of water to the Murray than NSW.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 2 seconds 1 m 2 s The Barmah Choke restricts the flow of water down the Murray ( Trudi Arter and Clint Jasper )

Does the plan affect what irrigators get?

There are fears the basin plan has increased the number of years New South Wales general security licences (the last in line to receive water) are on low or zero allocations.

Inflows to the Murray River overall have been decreasing — in the two decades to 1999, 12 years experienced above-average inflows; in the following two decades to 2019, only four years experienced above-average inflows.

With less rain, the variability of allocations, especially for those last in line to receive water, will also become more variable.

Last week, NSW Deputy Premier John Barilaro declared yet again that his state was interested in walking away from the basin plan.

"We need flexibility — in times of drought and what we're now absolutely enduring here in NSW, while South Australia continues to get water, that's just not fair," he said.

It's the Murray-Darling agreement, not the plan, that mandates 1,850 billion litres of water must flow to South Australia each year.

The plan sets targets for water recovery and a commitment from the states to put critical human needs first, but it does not determine how the states share water.

The Darling River in July, close to where millions of fish died last summer. ( Supplied: Geoff Looney )

Water licences explained: A water licence is the property-right equivalent of a bucket — it entitles the holder to a share of a state's water resource. When states are allocating water, they will up high-reliability buckets first, and general or low-reliability buckets last. The amount the bucket is filled depends on how much the state has to give out; in NSW for the past two seasons, general-security buckets have been empty.

Does the plan flush water out to sea?

NSW Water Minister Malinda Pavey told 7News in October that it was "heartbreaking" to see water flowing to the sea.

That report further claimed that "under the Murray-Darling Basin Plan, water from drought-crippled NSW" flows into South Australia, "filling lakes and wetlands before it's finally flushed out to sea".

The river is a drain, so the salt flowing from upstream accumulates in South Australia.

For decades, upstream states and South Australia have been arguing over salinity in the Murray River, and salinity, especially in the Lower Lakes, remains a divisive topic.

In the late 1970s, while debating salinity management, then-NSW water minister Lin Gordon said "if South Australia expects us to flush clean water down the Murray and deprive our own users, to wash the salt away, then they are expecting a bit much".

Speaking to Sky News last week, Mr Barilaro said: "If irrigators can't access water because there is a lack of water, then why are we putting priority around environmental flows?"

"You've got to build flexibility, everyone has to share the pain, so that's the environment and irrigators."

Last year, the CEHW did not meet the target set for allowing water to flow into the Coorong. ( ABC: Brittany Evins )

Flows of environmental water do not get priority over any release of water from the major storages.

The Commonwealth Environmental Water Holder (CEHW) has still had water available to use this season because it has entitlements from other states.

Murray-Darling Basin Authority (MDBA) data shows that between June 1 and last week, 27 per cent of the water released from Hume Dam was environmental water.

The basin plan sets targets for salinity levels through the river and for the Lower Lakes, and aims to export 2 million tonnes of salt from the Murray every year — but this is a long-term average.

Many protesters are not convinced that the lakes should be maintained as a freshwater ecosystem, and therefore find the notion of flushing salt from the system absurd.

At a minimum, the CEHW aims to get 650 gigalitres of water over the barrages at Goolwa to get fresh water into the Coorong and maintain its estuarine habitat.

Last year it failed to meet that target, and this year it remains below the target level.

A minimum of 2,166 billion litres has to held in storages before states can allocate water to licence holders. ( Supplied: MDBA )

Nearly all environmental and irrigation licences are the same

Much of the water recovered through voluntary direct buybacks is now held by the CEWH.

But even if those entitlements were still in the hands of irrigators, under current water-sharing rules and inflows, and the way NSW sets allocations, they would still be on 0 per cent.

If New South Wales walks away from the plan, or if it is scrapped, the Water Act would remain and with it the Commonwealth's mandate to meet its international obligations to maintain ecologically significant sights like the Coorong, so it could not sell the water back to irrigators.

While the plan sets out targets for water recovery, governments don't need the plan to recover water — it was happening prior to the plan under the Living Murray and Water4Rivers programs.

Further, the CEHW's roles are defined in the Water Act, not the basin plan, so scrapping the plan will neither eliminate environmental flow nor return the water it holds to farmers.

So what is flowing past the 'back gate'?

No-one would deny that watching water flow past a farming region like the Riverina, without the ability to use it, wasn't upsetting.

The overarching objective is to allow water to flow to its highest-value use — markets pre-date the basin plan, starting soon after state governments separated land rights and water rights.

Large corporate players with deep pockets have been entering the market ever since.

More than 70 per cent of water released from Hume Dam between June 1 and last week was for downstream human and irrigation users.

Water prices have been hitting Millennium Drought levels, and there are predictions they will surpass $1,000 per megalitre this summer.

It has priced out irrigators who have decided to rely entirely on the spot market for water.

Projections of water demand downstream of the Barmah Choke (the Murray River's narrowest stretch). The yellow and red lines show availability under average and drought conditions respectively. ( Supplied: DEWLP )

This year, the MDBA reported 620 gigalitres of water lost in "conveyance", the "cost" of moving water downstream, which rubs salt into the wounds of those going without water.

There is also deep suspicion right along the Murray about the influence investors have on prices, something the national competition watchdog is actively investigating.

But state governments have total control over land, including planning, and water rights.

Under state planning guidelines, the irrigation demand (for a fixed amount of water) has expanded unchecked for years, creating a situation where, in the near future, water demand will be more than what the river can deliver in all but the wettest years.

Victoria has put a pause on new developments, while New South Wales has not made any changes.

Hume Dam is the biggest dam in Australia. ( Supplied: WaterNSW )

So why can the plan?

There are plenty of reasons for irrigators and environmentalists to be angry at the plan, and the Murray-Darling Basin Authority, and plenty of reasons taxpayers across the country should be concerned about its implementation.

In a five-year review of the plan's implementation, the Productivity Commission (PC) issued a scathing list of warnings for basin governments, saying many of the projects designed to avoid the use of further water buybacks are so far behind schedule that taxpayers could be faced with a $560 million cost blowout to achieve the strict deadlines for water recovery.

The report was highly critical about the openness and transparency of the MDBA and the basin governments.

"Many stakeholders do not perceive that basin governments have taken the necessary time to listen and understand their concerns, to conduct the evidence-based analysis required to understand potential impacts and to explore options for managing these," the report said.

"They are also concerned that governments have been unwilling to listen and respond to community views, and they have not considered these views in decision making, or clearly communicated the reasons for their decisions.