The average size of new houses built in the ACT over the 2018-19 financial year was 250.8 square metres, according to the latest CommSec House Size trend report.

This is an increase of 10.1 per cent on the previous financial year.

Victoria has the second-largest houses at 246 square metres and Tasmania has the smallest at 178.5.

While houses in Canberra are the largest, the average apartment size is the lowest at 102 square metres - a 13 per cent decrease year-on-year.

Units are significantly lower than the national average of 128.8 square metres.

But new unit builds are far outweighing those of freestanding houses.

The proportion of houses built in the territory over the year was only 21 per cent, and CommSec chief economist Craig James said this is part of a wider trend spanning the better part of two decades.

"The general trend is that Canberra is building more apartments or non-houses," he said.

"Over time the proportion of houses being built in the ACT has come down significantly. If you go back probably a decade or two, about 60 or 70 per cent of homes built were freestanding homes.

"The ACT has some issues in terms of available land and there has to be a movement to urban consolidation rather than urban sprawl given the fact the ACT is pushing to its outer limits in terms of boundaries."

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The ACT government's 2018 planning strategy proposes an increase in density, with about 70 per cent of new housing stock in Canberra set to come from urban infill.

Mr James said Canberra's "transient population" could also be having an impact on apartment sizes.

"There is a more transient population in the ACT than what we have in some of the other states and territories where you have people coming in for sitting periods and the like," he said.

"They might be maintaining smaller digs in the nation's capital for two or three days a week whereas their main home is in another capital city."

The CommSec report, based off figures from the Australian Bureau of Statistics, found Australian house sizes are at a 17-year low.

The national average of 228.8 square metres is down 1.3 per cent over the financial year.

Seven years ago, Australia built the biggest houses in the world but the United States is now on top, with an average house size of 240.4 square metres.