Until its closure in 1986, the Union Carbide factory manufactured toxic chemicals, often overflowing uncontrolled releases of stormwater and wastewater into Homebush Bay.

"Since then it has all disappeared and the place has tidied up. It's just lovely to maintain the beautification of the area," said Ms Hill, a bushcare volunteer who has worked towards improving the health of the Parramatta River for the past 10 years.

She is one of more than 10,000 volunteers making a splash along the banks of Sydney's rivers, returning swimmers and fishers to some of the city's best watering holes.

The work of Sydney's volunteers was celebrated last week as part of National Volunteer Week. It is one of a series of "Source to Sea" events to demonstrate the improvements made to rivers around Sydney.

On Sunday a group paddled from Emu Plains to Yellomundee Regional Park after meeting with the Bass Sydney Fishing Group, whose ongoing work has led to a restored stretch of river at Emu Plains and improved conditions for bass fishing.

"It's really about celebrating all those people who give their time to improve the health of our rivers," said Vanessa Keyzer, regional landcare facilitator with Greater Sydney Local Land Services.

Sydney's major rivers include the Hawkesbury Nepean, Parramatta, Lane Cove, Cooks and Georges and all have varying degrees of health.

Best in show would be the Hawkesbury, said Gavin Birch, environmental chemist and adjunct professor at the University of Sydney, pointing to the work of Hornsby Shire Council

"It is the least contaminated. Hornsby have done a lot of excellent productive scientific work, which is very advanced."

In 2002 Hornsby Shire Council deployed the first of five buoys with monitoring probes in the Hawkesbury estuary. The buoys hook into the mobile network to report real-time water conditions directly to the community.

Mr Birch said an equivalent system is "absolutely needed" in Sydney Harbour.

"The most iconic waterway we use and we've only got one or two, but we are desperate for funding. I've been struggling for 10 years to get this system up and running."

Along the Parramatta River many hands have made lighter work, in the community-wide effort to make the entire river swimmable again by 2025, starting with the opening of Lake Parramatta last summer.

Thirteen councils sit within the Parramatta River catchment group and all have committed to tackling the two major polluters: sewer overflows and stormwater.

"We saw the success of programs cleaning up beaches and we thought we deserved the same for our river," said Parramatta Lord Mayor and chair of the Parramatta River Catchment Group Scott Lloyd.

A long way from becoming swimmable any time soon is the Cooks River, one of Australia's most densely urbanised catchments and relies heavily on the work of volunteer groups such as the Cooks River Alliance.

In 2013 a river health monitoring report card gave the river a "D+", however ongoing efforts by the volunteers mean small pockets of bushland and ecological communities remain.

Consistent monitoring also takes place along the Georges River, where community Riverkeeper programs regenerated 2785 square metres of riverside bushland, and collected more than 70,000kg of rubbish last year.

"The program has now collected five years of data on a number of important ecological indicators," said Georges River Combined Council's chairperson, Bankstown Councillor Naji Peter Najjar.

Since the start of the Georges River health program in 2009 more than 50 different community groups and 500 individuals have donated 4000 community hours to the program.

It all adds to the reputation of the river as a recreational destination, with swimming baths in at least four places, ample kayaking opportunities, and an abundance of fish, like mullet, flathead, silver brim and tailor.

Another recreational favourite is the Lane Cove River, to which Lane Cove Council has committed a number of future goals in its Coastal Zone Management Plan, including an improvement in water quality, estuary health and adaptation plans for the impact of climate change.