Canberra's drought has almost reached the worst-case scenario planned for by Icon Water, and at this point the city would need about 100 millimetres of rain to make an impact on dam levels, Icon Water managing director Ray Hezkial said.
Mr Hezkial said while the city is facing unprecedented dry conditions, they are prepared for drought events "significantly worse than the millennium drought".
Less than two years ago the current conditions being experienced were considered unlikely to occur. In September 2018, Icon Water told The Canberra Times the most-likely scenario didn't see Canberra going onto water restrictions for at least 20 years.
Just nine months later, that was revised and the possibility of water restrictions in 2020 was flagged. This was reiterated by the water corporation this week.
Mr Hezkial said the ACT, along with the rest of Australia, was facing unprecedented weather patterns in both extreme temperatures and reduced rainfall.
"2019 rainfall was 40 per cent below the long-term average, and average temperatures were 2.7 degrees above the long-term average," Mr Hezkial said.
"These unprecedented conditions have increased pressure on water supplies which may result in the need to bring in temporary water restrictions."
He said Icon Water's feasible worst-case scenario, called a design drought, was used to plan for the water supply.
"Whilst the intensity of this drought is close to the design drought, the duration would have to continue over an extended period to be as severe as what we have planned and are prepared for," Mr Hezkial said.
"Our team of analysts model literally thousands of potential scenarios to ensure our source water system can continue to meet the needs of our current and future population under a variable and changing climate."
The decision to move to water restrictions would be based on factors including the amount of water, the time of year, consumption patterns, weather forecasts, what's happening elsewhere in the region, and social and economic impacts associated with restrictions.
Mr Hezkial said Stage 1 restrictions, which are similar to the permanent water conservation measures already in place in the ACT, were introduced during the millennium drought when the ACT's total storage was 115GL.
"Due to the enlargement of the Cotter dam, we now have an additional 35 per cent storage capacity so although our overall dam levels are at 46.5 per cent, there is 130GL of water in storage."
He said for the rain to have a reasonable impact on dam levels, Canberra would need about 100 millimetres.
"In the longer term, it will take sustained rainfall over a longer period or a return to average rainfall conditions to significantly increase Canberra's water storage."
But, he said, Canberrans reduce their water consumption following any rain event, probably due to the effect is has on the watering of gardens and washing of cars.
"It is timely however, given the recent increase in both temperatures and consumption, to reinforce to the community that water is a precious resource and that we cannot be complacent. It is really important that we are conscious of our water usage and take action to conserve water wherever we can."
Mr Hezkial said the benefit of calling on the community to conserve water, rather than putting in place specific restrictions, was individuals could then make the choice of how to reduce their usage.
"For example, many Canberrans are very proud of their gardens, and imposing restrictions can reduce their ability to care for these spaces," he said.
"Without restrictions, they can instead choose to reduce their use in other ways, such as shorter showers and only washing their clothes on a full load."