The Abbott government's changes to age pension rules may have laid a trap for hundreds of thousands of retiring public servants.

The latest Finance Department reporting shows decisions by increasingly savvy Commonwealth workers are adding billions to the "unfunded debt" of public service pensioners. 

But in the wake of the latest figures, their public service retirees' group has warned that life for retiring pensioners is now a lot more complicated after reforms to age pension asset test rules.

Taxpayers will now need to find nearly $110 billion – almost 7 per cent of GDP – to pay the pensions of retired federal public servants, the Finance Department data says.

Finance's actuaries found a big increase in the number of public servants opting for indexed pensions from their CSS and PSS super funds rather than lump sum payouts, and a leading financial adviser told The Canberra Times it was the smart move to make.

But Superannuated Commonwealth Officers Association federal president Annette Barbetti​ sounded a note of caution on Monday, saying those taking indexed pensions might run into trouble if they wanted to claim the age pension, too.

The former Bureau of Statistics public servant warned that unwary public sector retirees might fall foul of the new beefed-up assets test regime, and that things could get "rather tricky".

"In view of the recent decisions to allow only 10 per cent of a defined-benefit pension to be excluded from the income test for the age pension, and to increase the age pension assets test taper rate, those who are planning to retire should seek financial advice which takes into account their personal financial circumstances," Dr Barbetti said.

The two super funds are closed to new entrants but have about 216,000 members still in the workforce, many of whom will soon face big retirement decisions.

About $2.2 billion has been added to the unfunded debt as a result of a higher-than-expected number of public servants rejecting a lump sum when they clear out their desk for the last time, and opting instead to take a pension.

But Dr Barbetti warned that retiring bureaucrats needed to look beyond the headline figures and get advice that considered their individual circumstances.

"It all depends on how much money you have, and it all depends on whether you are married or single. It's all terribly complicated," she said.

"But it could get rather tricky, and that's why people need financial advice."