Mr Barr said the new population figure, revealed amid the release of the latest census results, could explain the rising pressure on Canberra's hospitals and schools and competition in the local housing market.

The ACT Treasurer plans to hold urgent talks with his federal counterpart Jim Chalmers as he attempts to claw back some of the revenue.

A "frustrated" Mr Barr will also speak with the local assistant minister responsible for the census, Andrew Leigh, about addressing repeated problems with ACT population estimates, which he said had "significant implications" for how his government planned for and delivered services in the nation's capital.

New data published on Tuesday revealed the ACT's population had surged to 454,499 people on the night of the 2021 census, up 14.4 per cent from the last count in 2016.

That was the fastest rate of growth in the nation during that period.

It was also about 21,000 more than the Australian Bureau of Statistics' estimate for the ACT's resident population as of June last year.

The population figure is crucial for the ACT government as it's used as the benchmark for calculating how much it should receive in Commonwealth payments, including the GST.

Federal budget papers from March show the ACT's $1.46 billion share of the GST carve up this year was based on a population size of 437,000.

The ABS updated its population estimate to 453,320 following the release of the census results.

The bureau said there would be a variation between the population estimate and the census figure because they were difference measures. However, it didn't speculate on why the undercount had been so significant.

In an interview with The Canberra Times, Mr Barr said ACT Treasury was crunching the numbers to determine exactly how much revenue the ACT might have missed out on.

But Mr Barr expected the figure would be in the "hundreds of millions of dollars" over a five-year period.

He planned to speak with Mr Chalmers about a possible GST top-up payment at the end of this financial year to account for the revised population figure.

Mr Barr said the latest 2021 count appeared to be the third census in a row in which the population figure varied significantly from the ABS estimates, which are updated each quarter.

He suggested the major reason for the miscount was the "massive data gap" when it came to tracking interstate migration in Australia.

"Once people come into the country, where they set up in Australia - that's proving to be the most difficult [data to obtain]," he said.

"Using things like people's Medicare address is clearly not working."

Mr Barr said the ACT government would have to "recast" its budget and revenue projections in light of the new population figures.

Governments lean heavily on population data to plan for everything from new schools to hospital upgrades and housing developments.

Mr Barr said in recent years, ACT directorates had been instructed to factor in higher population numbers in their planning because of the bureau's record of underestimating how many people lived in Canberra.

"But usually it was about adding 10,000 more people ... this is 20,000 more - it's sort of double," he said.

"It's the equivalent of one Canberra suburb a year that's been undercounted."

The chief minister said the new population figure would strengthen the ACT's case for a greater share of federal infrastructure funding.

In a statement to The Canberra Times, the bureau said the census figures and the population estimate were two different measures.

"While the census aims to count every person in every dwelling in Australia on census night, the Estimated Resident Population [ERP] is a quarterly estimate of Australia's population," it said.

The bureau said the population estimate was the official measure, which was informed by the census results every five years.

"Having a comprehensive snapshot of the population down to a very small area level in Australia is critical for updating our every five years."