It is becoming clearer several former Howard government ministers including Peter Costello, Alexander Downer and Peter McGauran will quit federal Parliament soon.

Mr Costello and Mr Downer are backbenchers along with former immigration minister Kevin Andrews, ex-deputy prime minister Mark Vaile and former senior Nationals minister Peter McGauran.

Federal Opposition Leader Brendan Nelson says he has spoken to Mr Downer and Mr Costello about their plans but will not reveal if he has urged them to leave soon.

"I want them to make decisions which are in the interests of their electorates and of their families and then of themselves and our party, and I will expect that they will make those decisions very soon," he said.

He says if they retire he wants the by-elections held on the same day.

"If hypothetically speaking we do have a number of members who choose to retire some time over the course of this year, then it would be preferable that they did so on the one day," he said.