The ACT's population grew by almost 2 per cent this year.

The ACT's population grew by almost 2 per cent this year. Photo: Supplied

Watch out Tasmania, Canberra's almost 100 years old and, if you're not careful, could be taking your place as the sixth-most-populous (or third-least-populous) jurisdiction in Australia within a couple of decades.

In the lead-up to Canberra's centenary year, the ACT's population has exceeded projections by breaking through the 370,000 mark and is heading rapidly towards 400,000.

By contrast, a struggling economy has meant Tasmania has experienced only modest population growth.

Preliminary demographic figures for June showed that the ACT's population had grown by 1.9 per cent over the previous 12 months to about 374,700.

Tasmania's population grew by just 0.2 per cent, or 8000, to 512,000, the Australian Bureau of Statistics data showed.

If similar rates of growth were to continue, the ACT would have 400,000 residents by about 2016. By the early 2030s the territory would have more than 530,000 people and a bigger population than the island state.

But a return to more modest population growth levels would

reduce the ACT's chances of overtaking Tasmania.

Projections by the ACT government indicate the territory should reach a population of 400,000 by 2018 and 500,000 by 2043.

In 2008, projections prepared for the Tasmanian government showed the state's population could reach 550,000 by 2026.

In a recent interview, ACT Chief Minister Katy Gallagher said a larger territory cabinet and Legislative Assembly were needed to keep up with the demands of a growing population.

''I think by the time the city's [at] 400,000 and you're dealing with issues of that size, it's going to get too much,'' Ms Gallagher said.

''I think I'm trying to avoid the point where governance and government actually suffers.''

The government is considering a draft law which would enable a fifth minister to be appointed. The Legislative Assembly has had 17 members since self-government began in 1989 when the ACT had a population of about 275,000.

A working group chaired by ACT Electoral Commissioner Phil Green is looking at options for enlarging the Assembly. Tasmania has a 25-member House of Assembly and a 15-member Legislative Council.

Even if the ACT overtakes Tasmania in population, the territory could still find itself with fewer federal parliamentary representatives than the island state.

As an original state, Tasmania is guaranteed a minimum of five positions in the House of Representatives and the same number of senators as the other states.

Tasmania has five members of the House of Representatives and 12 senators. The ACT has two members in the House of Representatives and two senators.