There is plenty to celebrate about the historic rainfall bringing a dramatic end to Australia's unprecedented bushfire crisis.

It has brought welcome relief for firefighters and hope for farmers, but the record rainfall brings its own problems.

Queensland is grappling with overflowing dams while water authorities in New South Wales — who have also seen a major dam overflow — are dealing with the ash and debris from recent fires flowing into a number of critical water supplies.

So what does it mean for our water?

Is there fire and flood debris in dam water?

Yes. Most likely.

In the wake of the NSW bushfires, WaterNSW implemented additional water filters to reduce the impact of ash and fire-related debris in the dams.

The Warragamba Dam catchment area was hit hard this bushfire season. ( ABC News: Brendan Esposito )

And according to WaterNSW — and water experts — the volume of rain in NSW means debris from flood waters, ash, and other contaminants is impacting the quality of water supply in some areas.

"If you look at the upper reaches of Sydney's drinking water supply, along the Coxs River, we see a lot of mud and a lot of sediment and a lot of ash from those bushfires is now sitting in that waterway," said Professor Stuart Khan, an environmental engineer from the University of NSW.

"That can contain organic carbon [such as mud and sediment], and if there is organic carbon in the waterways, then bacteria that are naturally present will start to biodegrade."

It's an issue WaterNSW is monitoring closely, particularly in areas near the Green Wattle Creek fire, which was adjacent to the Warragamba Dam.

"Some debris, including residual ash, has washed into Lake Burragorang [in the Warragamba Dam area]," a WaterNSW spokesman said.

"WaterNSW and Sydney Water are keeping a close watch on any changes to water quality."

An area of the Warragamba Dam hit with runoff from the bushfire zone. ( ABC News: Brendan Esposito )

They are also able to prioritise the cleanest water in the dam to enter the filtration plant.

Victoria's Thomson Catchment Complex faced a similar threat following fires earlier this month.

And although authorities are still monitoring the situation, a Melbourne Water spokesman said the bushfires around Victoria had not impacted Melbourne's water supply catchments or assets.

With so much rain, why water restrictions?

Dr Khan said when big rain events happened in some areas, the water quality was impacted so much it was not easy, or sometimes even possible, to produce clean drinking water.

This the case in the Bega Valley, in south-east NSW.

Water is being trucked in at a cost of around $30,000 a day, according to the Bega Valley Shire Council, despite the Brogo dam currently being at 110 per cent capacity.

That's because the turbidity — or the mud and debris in the water — is too thick for its water filtration system to cope.

"[It means] the Brogo River supply has been isolated and emergency measures put in place," Bega Valley Shire Council water and sewer manager Chris Best said.

"Brogo Dam rose from 10 per cent to overflowing in just one day — the water flowing into the dam is full of sediment, ash, soot and debris."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 49 seconds 49 s Residents in Bega rejoice as floodwaters surge down a dry riverbed. ( ABC News )

Brisbane faced a similar water quality crisis in January 2013.

Major flooding created a drinking water shortage because the water filtration system couldn't keep up with the volume of water, meaning Brisbane almost ran out of drinking water during a flood.

Mike Foster from Seqwater said taps in the city's outer suburbs that year very nearly ran dry.

"We were getting close to almost a matter of hours," he said. "It got pretty tight."

Is the water safe to drink?

Largely yes, but it depends where you are.

WaterNSW said some deterioration in water quality was expected after significant rainfall and acted to manage it following the state's bushfire crisis.

That included the installation of three booms and dam curtains in the gorge at Warragamba Dam as a precautionary measure.

A dam curtain works a bit like a shark net, but instead of blocking deadly marine life, it prevents potentially harmful sediment from reaching the dam walls where our water begins the journey through filtration processes.

"They're anchored on either side of the reservoir and they have a curtain that drops down two or three metres below the water surface," Dr Khan said.

"The idea there is that the sediment is mostly floating on the surface so they will try and hold back that sediment, or the top one or two metres of water, rather than letting it flow down to the dam wall."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 35 seconds 35 s Heavy rain forces water to cascade out of Nepean Dam

Above the water line, there's a different strategy.

"(Authorities) can put in place erosion barriers which in many cases look a bit like fencing with a mesh material to just try and hold back some of the soil and sticks and everything else that might get washed down into waterways" he said.

In Sydney, multiple river and dam systems mean authorities can find other sources of water to draw on in the event of an emergency, but in regional areas where there is only one waterway, other actions are likely to be taken, such as trucking in water or turning to another water source.

"At the moment we're not seeing any impacts to Sydney's drinking water supply and we probably won't," Dr Khan said.

"There are some systems that are much more protected and much more resilient than others."

What about that cyclone off the east coast?

In south-east Queensland, the dams have seen as much as a 25 per cent increase in the past week and they're expecting more rainfall this weekend as a result of cyclone Uesi.

Lake Macdonald on the Sunshine Coast is at 104.5 per cent capacity, while Gold Creek, one of the smaller dams in the south-east Queensland network, is at 146 per cent, causing them to overflow.

But south-east Queensland is prepared for the wet season, according to Seqwater.

"We were sort of getting most of our large events on the Gold Coast and the Sunshine Coast," Mr Foster said.

"While a number of our dams across the region will hit capacity and [are] spilling, we've come through to date unscathed and the water supply is continuing unabated."