The sight of more than a million fish floating belly up on the Darling River at Menindee has thrown doubt over the management of the Murray-Darling Basin.

Experts say irrigators are taking too much water from the system, and the Murray-Darling Basin Authority has mismanaged water flows.

But New South Wales Water Minister Niall Blair says drought is to blame.

With more fish likely to die, here's what we know about the mass deaths and what some independent experts have had to say.

Where are fish dying?

A million fish were found dead at Menindee Lakes last week.

It's a series of seven lakes fed by the Darling River, about 90 kilometres south-east of Broken Hill in western New South Wales.

Map The Menindee Lakes, south of Broken Hill, are connected to the Darling River.

It's believed to be one of the largest fish kills ever recorded in Australia.

Then a smaller kill of about 60 fish was reported at Lake Hume yesterday, on the NSW-Victoria border. But the cause of that kill is still unclear.

What killed the fish?

A variety of factors were at play at Menindee. Water levels were very low, the system had stopped flowing, and temperatures were high after a long spell of hot weather.

This created ideal conditions for blue-green algae (cyanobacteria) to grow, and it "bloomed" out of control.

But it wasn't the algae that killed the fish.

A cold front hit the region, which dropped the water temperature in the river, killing the algal bloom.

The bacteria that feeds on dying algae then exploded out of control, and sucked all the oxygen from the water.

When the oxygen levels dropped too low, the fish drowned.

So who or what is to blame?

The blame game began almost immediately after the Menindee fish kill was reported.

Sorry, this video has expired Rob McBride and Dick Arnold made this emotional plea to politicians about water management.

Farmers Rob McBride and Dick Arnold, whose video of dead Murray cod went viral, pointed the finger at cotton growers and politicians.

But others blame mismanagement by the Murray-Darling Basin Authority (MDBA), and the NSW Government blames drought.

The big question is: why was the river in such a state that a blue-green algae outbreak of this scale could occur?

Here's what four experts working in the fields of river ecology, policy, management and economics told us.

1. Expert in water economics, environment and policy

Adjunct Professor John Williams from the Australian National University said you couldn't blame the drought.

"To manage a river you've got to be able to manage it through a drought without killing all the fish," Professor Williams said.

"We didn't put enough environmental water aside, and then we've continually eroded the little we did allocate with the recent amendments both in the north and to the south."

Low flows frequently see stretches of the Darling river dry up. ( Emma Brown )

Environmental water is water set aside to be released into the river system when needed.

In 2018, the Turnbull government won support from Labor to amend the amount of environmental water allocated to the system, while the Greens and some senators were opposed.

The amendments cut 605 billion litres a year that were allocated from the southern basin's environmental water flows, and 70 billion litres a year from the northern basin's flows.

Professor Williams said if more environmental water was allocated to the system, it could be used in times of drought to help flush the system, reduce nutrient levels, help drop water temperatures and oxygenate the water.

"Yes, it is hard to manage rivers like the Darling through drought, but that's Australia. If you haven't got a management plan that can manage the water through drought in the Darling, you haven't got a plan," he said.

"We're taking a hell of a lot of water out. We had good flows 18 months ago.

"We want working rivers, we want irrigation, but we need to know how much we can take and regulate it pretty strongly."

2. Expert in conservation biology, wetland and river management

Professor Richard Kingsford from the University of New South Wales said farmers and irrigators were suffering from the drought, but water management was a big issue.

"Certainly the drought is a contributing factor. The bigger issue is that this has been coming for a long time in the Darling," he said.

"Over the last 20 or 30 years, we've reduced flows coming into the Menindee lakes from upstream and down the Darling by almost 50 per cent. And it means there's less water in the river than there used to be."

As well as extracting water from the river, licences allow some irrigators to capture overland flows.

Overland-flow capture means diverting rainwater into storage before it reaches the river, which in turn leads to less water entering the system.

"Some cotton growers in the Darling River tributaries have managed to capture some of the water in the recent rains that have occurred, and that's part of the licencing system that allows them to do that, to harvest those flows," he said.

Professor Kingsford says it could be some time before the fish population bounces back. ( Facebook: Debbie Newitt )

3. Expert in water policy reform

Professor Michael Young from the University of Adelaide said the Murray-Darling Basin Authority had failed to plan for lean times.

"We've put a lot of effort into debating what is called the 'sustainable diversion limits', which is working out the maximum amount that can be taken when the tank is full," he said.

"We've put very, very little effort into working out how to manage times of low flow and who's responsible for that."

In the United Kingdom, there is a policy called "hands-off flow", where water is released at the top of a system and that water cannot be extracted as it works its way downstream.

But in Australia, things are different.

"In much of the Darling at the moment we don't have mechanisms in place to shepherd water through the system," Professor Young said.

"The licences people hold are often a function of the height of the river. If one person leaves water in the river, the next person says, 'thank you, I'll take some more'."

4. Expert in ecology, management and restoration of aquatic ecosystems

Professor Robyn Watts from Charles Sturt University said that drought, climate change and mismanagement had all contributed to the state of the river.

"There's a lot of complexity around this fish kill," she said.

"It's hard to know if that could be avoided because there's so much complexity around the Menindee Lakes system in terms of who's been taking water upstream and whether it's been taken legally."

But she said there were things that could be done in the short term to avoid more fish kills.

Better water management could have avoided the mass fish kill at Menindee according to experts. ( Supplied )

During previous events, locals have improvised their own aerator systems, pumping oxygen into affected waterholes and moving cooler, deep water to the surface.

"Where these refuges were created … we got the most adults and the most fish larvae," she said.

Yesterday, the New South Wales Government announced it would be installing aerators at a number of sites across the state.