Eager to see a reporter and someone with a camera, the pair waved us past the boom gates and urged us to take as long as it took to make sure we got the message out.

Burrinjuck Lake has gone green.

Since Australia Day weekend, blue-green algal cells have bloomed and spread across the popular water-skiing and fishing spot, thriving in the warm summer waters.

Located about 55 kilometres south-west of Yass, and twice the volume of Sydney Harbour at capacity, the lake flows into Burrinjuck Dam, supplying water to towns including Jugiong, Gundagai and Wagga.

It feeds from the Goodradigbee and Yass Rivers, as well as Canberra's Murrumbidgee.

The NSW government has issued a red alert for the lake, advising visitors through the WaterNSW website that the water is not safe for swimming or water-skiing.

The lake is the park's main drawcard, home to Murray cod, yellowbelly, silver perch, brown and rainbow trout, redfin and European carp.

The blue-green algae which has taken it over is a natural occurrence, preferring warm, calm and nutrient-rich conditions to grow in the excessive numbers known as blooms.

The smelly bacteria can cause rashes on the skin and irritate the eyes if it comes into contact with them. It can make both humans and animals sick, and boiling it won't make it safe.

Despite the red alert, Mrs Brind and Mrs Day-Tomlinson have had to turn families with boats away at the gates. They tell me they're eager to avoid delivering more disappointment to water-skiers and swimmers trekking down for Canberra Day weekend.

The pair have taken it upon themselves to begin emailing out warnings to guests, and thankfully, the result has been more trips being postponed this week than new bookings.

The Burrinjuck outbreak is one of several across NSW, including multiple blooms in the lower Murray, which have been on red alert for a number of weeks.

A spokesperson for WaterNSW said the algal blooms were somewhat unusual for Burrinjuck given the dam had recently received large inflows, is quite full and is located in a cooler part of the state.

Reduced vegetation cover following extreme dry conditions may have resulted in inflows from 2020 transporting water from nutrient-dense catchment areas, the spokesperson said. Water treatment processes are in place to remove the algae to make it safe to drink.

According to Canberra water providers Icon Water, agriculture in the Googong and Murrumbidgee catchments increase the nutrient levels in the waterways, making the water sources more susceptible to algal blooms.

While Icon has detected higher than usual levels of algae in its Googong Dam water this summer, it has been sourcing Canberra's water from the Cotter Dam, which is 100 per cent full and has not seen an increase in algae.

While some of that water comes from the Murrumbidgee, Icon is not the water manager for the river. Rather, blue-green algae in the river and Canberra's popular swimming spots is monitored by the ACT government, which is responsible for releasing information to recreational water users.

At present, spots along the Murrumbidgee in Canberra are closed to swimmers include Casuarina Sands, Tharwa Bridge and Uriarra East and West.

The City Services website lists "bacteria" as the reason for their closures.

An ACT government spokesperson said spots close for a variety of reasons, not just algae.

"Recreational waters generally contain a mixture of pathogenic and non-pathogenic organisms," the spokesperson said.

"These pathogens may be derived from other bathers, sewage effluent overflows, domestic, rural and native animals. The pathogens include viruses, bacteria, protozoa and helminths.

"To protect the community, the ACT government inspects, collects and analyses samples from Canberra's lakes, ponds and rivers weekly.

"A recreational water area may be closed if deemed that there is a potential risk to public health."

CSIRO research scientist Grant Douglas said it was likely there would be more outbreaks of blue-green algae - also known as cyanobacteria - in the future, as global warming heats up our water sources.

Mr Douglas said there was no single driver for the blooms, but warmer temperatures, still waters, lots of nutrients and the fact that cyanobacteria was "the great survivor" meant it bloomed prolifically, often forming scums on the surface of water.

"The specific effect of climate change is that if we do get reduced flows into rivers and into dams, you don't get the flushing of the nutrients through the system and you also don't get the mixing of the water," he said.

Mr Douglas said this can lead to stratification; a warmer upper layer and a cooler lower layer that tended to favour the proliferation of blue-green algae.

To get rid of the algae, the warm, nutrient-dense, still-water recipe needs to be reversed.

"Essentially something has to either limit the growth of the blooms so that they run out of nutrients to feed on or there has to be something that disrupts the system sufficiently. Often that doesn't happen and these blooms will persist for some time," Mr Douglas said.

He said the algae can produce toxins which are potentially harmful to a variety of animals but the real damage is caused when they start to break down.

"When these blooms collapse - so when they are disturbed or die - the majority of the algae will then sink to the bottom which will then consume a lot of the oxygen in the water," Mr Douglas said.

"Once you consume all the oxygen in the water that can lead to fish kills because they don't have any to breathe."

Water NSW said it was impossible to predict how long the algae will remain at high levels or where it may spread to. Regular monitoring will continue, and the alert will be lifted as soon as the algae dissipate.

Mrs Brind and Mrs Day-Tomlinson say they share a concern that the worst algae outbreak they've seen in more than a decade has gone largely unnoticed.

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"We've had guys coming here for 20 or 30 years, and they say they've never seen the dam like this ever," Mrs Brind said.

"If it gets on your skin it irritates your skin to the point where you've got to have a shower, you break out in rashes.

"If you get a gutful of the water out there, instantly mate, you're vomiting and have diarrhea."

At almost 70 per cent capacity - well above the state's average which is closer to 50 per cent - higher than average rainfall has meant Burrinjuck is the fullest it's been in years.

"And yet, you can't even get out there and enjoy it," Mrs Day-Tomlinson said.

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