As Melbourne's water-storage levels dropped to 28.2per cent last week, the Monash Alliance - a group representing VicRoads, Abigroup Contractors and Sinclair Knight Merz - was close to completing a "rain gardens" project to filter stormwater run-off from the Monash Freeway.

The project uses an emerging bio-filtration technology to filter out nutrients and other pollutants.

Annually, it is expected to remove 21kilograms of nitrogen, 4.5kilograms of phosphorus, plus hydrocarbons and E.coli, from stormwater going into the lower reaches of the Yarra River through Gardiners Creek.

Next to Gardiners Creek, by the Toorak Road outbound off ramp, the garden will treat water from recently expanded sections of the freeway.

Monash Alliance project manager Peter Lellyett said

water from the freeway going into the lower reaches of the Yarra River would have fewer pollutants.

"While we're here to achieve a project that provides better roads, there is a right way to do that which is ensuring it's sustainable and makes a positive difference to the environment."

Mr Lellyett said water pumped to the garden would be filtered through soil where plant roots would take up nutrients and trap pollutants.

Treated stormwater would be collected in pipes at the base before entering the stormwater system to be discharged into Gardiners Creek.

Mr Lellyett said the alliance used non-potable water - including stormwater and recycled water - during the upgrade.

Cleaner water was used during construction and less-clean water was used to suppress dust and water vegetation.

"We've put in a lot of work to ensure we don't add [more] load to Melbourne's water system than is necessary," he said.

Monash University's Institute of Sustainable Water deputy director, Ana Deletic, said it was important to treat stormwater from highways because it was significantly polluted.

Professor Deletic said more stormwater ran down drains each year than Melbourne's annual water consumption.

She that if utilised more widely, stormwater could solve Melbourne's water crisis.

"It's a great initiative that costs very little. It makes sense that, with such a big investment in our roads, they do that little bit extra to capture stormwater."

The $250,000 rain garden will be completed this month.