Restricting the amount of greenery growing beside the state's waterways will have a detrimental effect on the environment while failing to meet its goal of reducing bushfire threats, top waterway ecosystem scientists have warned.

Announced in January, the Andrews government policy limits vegetation along rivers and streams to a 10-metre-wide area each side of the waterway.

While initially targeting a lowland area between the Dandenongs and the Yarra Ranges, the six water scientists fear the policy will set a dangerous precedent for the management of the state's waterways and potentially the city's water quality.

"The policy was heralded to better manage bushfire risk while effectively managing environmental values. It does neither," Melbourne University wetlands ecologist Joe Greet said. "It's very poor policy from a conservation perspective."

Speaking on behalf of concerned water scientists Yung En Chee, Chris Walsh, Geoff Vietz, Fiona Ede and Tim Fletcher, Dr Greet said all the evidence showed a 10-metre limit would be "manifestly inadequate" for conservation.

Instead a 20- to 38-metre buffer had been recommended to government in the past, based on research.

Dr Greet said this was because vegetation beside waterways not only served as wildlife habitat, it also contributed to stream health and water quality by filtering agricultural runoff and providing a buffer between cleared and farmed land.

He said studies had shown rivers changed their course when waterside vegetation narrowed, meaning wider vegetation areas would better protect infrastructure such as fences and roads by limiting erosion.

"We are concerned that this is bad policy in response to community concerns about bushfire risks," he said. "That should be acknowledged ... but the current government-led research doesn't support a strong link between (waterside) vegetation and bushfire risk."

The new rule will apply to the Yellingbo Conservation Area which includes 150 kilometres of streams and 24 kilometres of the Yarra River.

The site had been identified as a revegetation area to expand the available habitat for two of the state's faunal emblems; the Leadbeater possum and the helmeted honeyeater. Both are critically endangered, with wild populations confined to the tiny Yellingbo Nature Conservation Reserve on Melbourne's northern doorstep.

While the nature reserve will be exempt from the new rule, conservationists are concerned the possum and bird could be wiped out if a bushfire goes through the reserve. Spreading wild populations over different locations reduces that risk, they argue.

President of Friends of the Helmeted Honeyeater Bob Anderson went further and said a waterside buffer of at least 70 metres wide was needed for it to be suitable habitat. And between such areas, "corridors" were needed so the birds and the possums could move for feeding and nesting.

"Otherwise it will limit the ability of the birds and the possums to move out of Yellingbo," Mr Anderson said. "By restricting them to the Yellingbo Nature Conservation Reserve, it's like holding a zoo population."

However the Yarra Waterways Group, which represents more than 200 residents in the Warburton district, argues widening the vegetation areas beside waterways would increase the bushfire risk and compromise the safety of residents.

Chairman Rick Houlihan said restricting waterside vegetation to 10 metres wide made it easier to access the land when fighting fires.

"You can fight any fire that is 10 metres deep… but if a fire is on a reserve up to 200 metres wide, the only way you can fight it is with aerial support as you can't get the fire trucks in."

He said 10 metres was a compromise for the local community, which given the 2009 and 1983 fires was extremely "fire conscious".

"Our main thrust is to keep our region, its people and visitors safe," he said.