There is concern wild horses in Kosciuszko National Park are contributing to blue-green algae in dams and streams in the New South Wales Snowy Mountains.

Key points: Fishermen in the Snowy Mountains have reported visible blue-green algae in the high country waterways and have reported seeing platypus in distress

Fishermen in the Snowy Mountains have reported visible blue-green algae in the high country waterways and have reported seeing platypus in distress Experts say that a large number of horses accessing waterways may be a contributing factor to algal growth

Experts say that a large number of horses accessing waterways may be a contributing factor to algal growth Small volumes of water in recent may also have led to conditions where blue-green algae can flourish

Colin Sinclair has been fishing in the high country since the 1960s and said he has only seen blue-green algae the area in the last five to seven years.

He said it is not killing fish, but is visible.

"It can send all the rocks just a lime green and you can pick the trout out pretty easy going across it," he said.

Mr Sinclair attributes the blue-green algae to very high summer temperatures, especially over the 2018/19 summer, and questioned whether brumbies are enhancing the bacteria in the water.

"I'm no scientist, but brumby numbers have been pretty thick up there for the last five or seven years and now you're seeing the algae there," he said.

"I love seeing the brumbies, but I do not like seeing brumbies in plague proportions like we're seeing at the moment."

Rod Whiteway, secretary of the Monaro Acclimatisation Society, a recreational fishing and conservation group, said the Currango Creek used to be a pristine stream filled with healthy wildlife but its banks have been completely trampled in by brumbies, taking away fish habitat.

"The deep holes have been silted up, there's hardly any water weeds — I can't imagine a platypus living in any of those holes, I can't imagine a large trout living in any of those holes," Mr Whiteway.

Associate Professor in Water Science at the University of Canberra, Fiona Dyer believes the number of brumbies is a factor.

"A large number of horses accessing waterways within that catchment will contribute nutrients to the dam and they will be nutrients that are then available to support algal growth, but they won't be the only contributor," she said.

Dr Dyer said heat and low water flows were other major factors.

Concerns algae impacting local platypus

Mr Whiteway said he recently saw a distressed platypus in an algal bloom in the upper Murrumbidgee River near Tantangara Dam and wonders whether it was affected by the bacteria.

"I walked right up to it and it was trying to swim away and swim under water, but just spiralling away," he said.

"Over the next 10 minutes, it was washed by the current, still spiralling, about a hundred metres downstream.

"I do wonder if I'd been drinking the water, I might have suffered the same fate as the platypus."

There are concerns about platypus colonies in the Snowy Mountains with blue green algae appearing in waterways. ( Supplied: Sharon Wormleaton )

Dr Dyer said it is difficult to say if the algae is causing this distress.

"I guess I'd be cautious about making inferences that the platypus have been affected by the blue-green algae in that system on the basis of a single observation, but it'd certainly be worth looking into to find what's affected the platypus," she said.

NSW National Parks and Wildlife Service (NPWS) said the agency checked the location twice without sighting a platypus.

A spokesperson said reports of possible blue-green algae in Kosciuszko National Park are very rare but National Parks is concerned about any water quality issues that might impact on wildlife, including platypus.

Snowy Hydro has confirmed there is algae in the Murrumbidgee River near Tantangara Dam.

Record low flows also a factor: expert

Dr Dyer is on the Snowy Advisory Committee which was established last year to advise on environmental flows in the Snowy and Montane Rivers affected by the Snowy Hydro system.

She said releases from Tantangara Dam into the Murrumbidgee River over the last five months have been at a very minimal level.

"The amount of water is a function of the inflows to the Murray Darling Basin system last year," Dr Dyer said.

"And so the amounts of water that are available in that system this year are very small, some of the smallest on record."

Water NSW, the government agency responsible for river water quality, said in a statement that blue-green algae occurred wherever conditions were favourable.