The water holds about 30 years of pollutants and sediment, ACT Healthy Waterways project manager Justin Foley said.

Downstream, as the drought takes hold across NSW the impact of water quality on growers is becoming more apparent.

Leeton area farmer Stuart Buller pulls water from the Murrumbidgee to farm rice and livestock. He lost three sheep in the last outbreak of blue-green algae in his area.

“You’ve got to keep checking your sheep daily, make sure it’s not affecting them,” Mr Buller said.

“Anything they do to eliminate this problem has got to be a good thing."

Canberra is the largest urban settlement in the Murray Darling Basin, hence the millions of dollars of federal government investment in the territory’s waterways to improve the quality.

But it’s not just for those downstream like Mr Buller. Canberrans will benefit from the cleaner water too.

“We’re treating the issue at a local scale but it’s delivering a much bigger outcome,” Mr Foley said.

“There is a genuine concern from a health perspective. It has an impact on fauna and it has a significant impact on our ability to use our lakes.”

“We’ve got a great landscape in the ACT, we really want people to go out and use the lakes, but while we’ve got algal outbreaks, we can’t do that.”

In the 10 years to 2017, the Lake Tuggeranong was closed for an average of 93 days each year due to algal outbreaks.

Mr Foley said despite good intentions all those years ago when Canberra was being developed, the philosophy around the management of waterways was primarily to manage floods.

“They were set up correctly for that, with lakes and ponds set up as sediment traps. There were well-meaning people at the time dealing with the issue as they understood it.”

However, over time and taking into account the impact of urbanisation has meant a change is overdue.

“What we’re now trying to do is slow the pollutants, make sure they're processed. I don't think anyone had the information necessarily at the time to do things differently, but it is a big issue now that we need to deal with,” Mr Foley said.

As part of the Healthy Waterways project, the construction of what’s been dubbed the largest rain garden in the southern hemisphere is now complete.

The plants that will adorn the garden were specially selected for their ability to survive the harsh Canberra climate, and draw nutrients out of the water. They will be planted in spring to give them the best chance at survival.

A two-year establishment period will follow before the giant water filter is ready to take the required flow of Canberra’s stormwater.

“You want to make sure you’ve got plants that are effective at treating pollutants,” Mr Foley said.

“There’s a bit of an art to plant selection.” he said.

A rain garden has plants on top and a filtration system below, to allow some of the water flowing from the stormwater system to be cleaned before it ends up in Lake Tuggeranong, and then the Murrumbidgee.

To improve water quality there are 12 sites under construction across Canberra, including ponds, rain gardens and wetlands. A further seven are in the final stages of design, and three more are going through the development application phase.

“You decide what type of asset you’re going to use based on what’s coming in through the catchments,” Mr Foley said.

“It depends on the catchment, it depends on the pollutants you

might treat, and it depends on the space you have available as to what you might use.”

The rain garden in Monash will target nitrogen, phosphorus and suspended sediments.

“Phosphorus is the key to blue green algae,” Mr Foley said.

“If we can limit that, we can limit the blue green algae. Nitrogen drives a whole lot of algal processes as well. They’re the things you want on your garden to make your plants grow, but unfortunately if you get them in the waterways, the algae grows.”

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