Cotter Dam reached capacity this week for the first time in two years, and the smaller Bendora Dam followed suit on Monday night after a downpour which brought 16mm to Canberra's gauges.

Canberra's four dams are currently at a combined capacity of 86.6 per cent and Icon Water analysts expect they will reach to 90 to 100 per cent capacity by December.

"The water that spills from the Bendora Dam is captured in the Cotter River, and makes its way downstream to be captured by the Cotter Dam," an Icon Water spokeswoman said.

"The increase in river flow along with rainfall has contributed to Cotter Dam reaching 100 per cent."

Following a record hot and dry summer, storage levels dropped below 45 per cent in February this year.

"With above average rainfall this winter, Canberra's water levels have recovered significantly," the spokeswoman said.

The Bureau of Meteorology declared a La Nina event last week, which was predicted to bring cooler days and wet weather in spring and the start of summer.

Rainfall in August went far above average this year, with 105 millimetres recorded by Icon Water compared to the monthly average of 44mm.

A further 40mm was recorded in September, below the monthly average of 52mm.

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A further 40mm was recorded in September, below the monthly average of 52mm.

The spokeswoman said based on current modelling combined dam levels would likely reach 90 per cent by December, and potentially hit capacity.

"Weather has the greatest influence on our water storage position - with consumption also a contributing factor," the spokeswoman said.

"BoM declared this week that a La Nina event is officially underway which makes increased rainfall over spring a high probability."

Cotter Dam hit 100 per cent capacity in 2018 and Canberra's water storage has not been at full capacity since 2016.

Googong Dam is currently at 84 per cent and Corin Dam at 73 per cent capacity.

The effects of La Nina will soon be felt in the ACT with a rainy week forecast by the Bureau of Meteorology.

Up to five millimetres is expected to fall on Tuesday and another 10mm could hit on Wednesday.

The BoM expect Thursday will bring a deluge of 10 to 20mm and a thunderstorm is forecast.

The Icon Water spokeswoman said despite the bountiful water stores, residents should maintain water-saving habits.

"While we have seen an increase in rainfall this year and improved water security, Australia is the second driest continent on Earth," she said.

"With the impacts of climate change, it's expected that we will experience significant drought again in our future, so it's important that we work to maintain our water wise habits so that we can protect our water supply for generations to come."

Permanent water conservation rules are in place in the ACT. These measures introduced in 2010 include watering gardens when evaporation is least impactful and cleaning cars using a high-pressure hose.

COTTER DAM: 100 per cent, 76.2 gigalitres

BENDORA DAM: 100 per cent, 11.45 gigalitres

GOOGONG DAM: 84.93 per cent, 101.29 gigalitres

CORIN DAM: 73.09 per cent, 51.74 gigalitres

Combined volume: 86.67 per cent, 240.80 gigalitres