State population decreased by 476 to 540,839 in the September quarter last year, Australian Bureau of Statistics estimates released on Thursday said.

It was the second consecutive quarterly decline, following a net loss of 173 residents in the three months to the end of June.

They were they first quarters since 2015 when state population fell.

It will not be known for some months whether the reopening of state borders in December and potentially higher immigration from overseas sent the population trend back into growth.

Tasmania lost 513 more people to the mainland than it gained from the mainland in the September quarter.

It also lost 160 more people overseas than it gained from overseas - the fifth negative quarter of the past six on that measure - as inward immigration was slashed by coronavirus and related restrictions.

The only growth component in the September quarter was natural increase (births less deaths).

That added 197 Tasmanians, down from 364 in the previous quarter.

Demographer Lisa Denny said the September quarter was when New South Wales and Victoria were in lockdown due to the Delta coronavirus strain and Premier Peter Gutwein said: "Tasmania is going to experience population growth well above the Treasury forecasts because interstate migrants are knocking on the door and knocking loudly."

Dr Denny said: "In one way he was right."

"People from interstate were knocking loudly on our door and more people arrived in Tasmania from interstate during the September quarter than ever before

"But he was wrong in claiming that Tasmania is going to experience population growth well above the Treasury forecasts because he failed to consider that more people may also be leaving Tasmania to live interstate than ever before.

"And this is what is happening.

"Over 5200 people left Tasmania to live interstate during that quarter, the highest quarterly number of people on record, and nearly 1000 more than the previous highest number, which was for the June 2021 quarter, also during the pandemic."

National population increased by about 12,100 people in the quarter, thanks to natural increase exceeding a net loss of 19,900 people to other countries.

.