Before a cooler change forecast for Tuesday, Canberra also set an unenviable summer record on Monday, for the first time registering its sixth day in a row of temperatures reaching or exceeding 37 degrees.

Volunteers have found dead fish floating in the Giralang Pond off William Slim Drive in the city’s north and reported high temperatures in rivers and lakes across the ACT.

In its monthly climate report issued on Monday the Australian Bureau of Meteorology said just 4.8millimetres of rain was recorded at the monitoring station at Canberra Airport in January, less than one-10th of the average of 58.5millimetres.

It was the fifth driest January on record for Canberra and the driest since 1985.

The bureau said it was also the first time Canberra recorded three days of at least 40 degrees in January and the capital had recorded just 13 days above 40 degrees since 1939, when records began.

There were nine days hotter than 37 degrees in the first month of this year, smashing the previous record of five days in January 2013 and far exceeding the average of one day.

The capital’s average maximum temperature was 31.6degrees, 3.6degrees above the average but slightly below the 32.3-degree record set in January last year.

Ginninderra Catchment Group Waterwatch co-ordinator Damon Cusack said dead carp had been found floating in the Giralang Pond over the past week, after hot water temperatures combined with other factors had caused a "black water event", a lack of oxygen in the water.

Waterwatch is a national program in which volunteers regularly check the water quality of wetlands, rivers, lakes, creeks and stormwater drains.

"This is the first time we’ve seen this sort of event in at least four or five years, and probably longer," Mr Cusack said.

"The water levels are very low in Ginninderra Creek, so there’s been some very poor water quality throughout our system over the past month or so."

Southern ACT Catchment Group Waterwatch co-ordinator Martin Lind said urban waterways in Tuggeranong were becoming increasingly stagnant.

One volunteer had measured a water temperature of 31 degrees in the Murrumbidgee River near Tharwa on a 35-degree day, another had recorded a water temperature of 28 degrees at Isabella Pond, part of Lake Tuggeranong on a 29-degree day, he said.

Mr Lind said hot water was dangerous for local wildlife. "What happens when the water gets hot is the oxygen disappears; water temperature alone will kill a lot of fish and once you add that to the oxygen leaving the water it becomes quite deadly for the wildlife, for fish and all the things they eat," he said.

Bureau climatologist Acacia Pepler said temperatures in Australia had warmed by about a degree over the past century, causing an increased frequency in hot days and heatwaves. "We have had heatwaves in the past, a couple of big ones for southern NSW in 1908 and 1939, but we’re definitely seeing these records being broken at a faster rate than you would expect if temperatures hadn’t warmed up by a degree," she said.

Ms Pepler said while it had been a hot start to 2014, it had not topped January last year, the warmest single month for Australia on record.

But the outlook for February to April suggested a likely return to more average weather patterns.