Opposition Leader Tony Abbott has imposed a travel ban on Coalition MPs heading overseas, saying they need to be prepared for an election at any time.

Prime Minister Julia Gillard has already announced September 14 as polling day, but Mr Abbott believes she will call the election sooner if it suits Labor's interests.

"No overseas trips," Mr Abbott told colleagues at a meeting in Canberra this morning. "Be ready, be visible and be in Australia.

"Each one of us has to be a person of the people and for the people in the lead up to polling day."

He praised MPs for their discipline, but warned them against taking an election victory for granted.

"We are the underdog. Oppositions always are," Mr Abbott said.

Even though the official election campaign has not yet begun, the Coalition and Labor are already trading blows over the release of policy costings.

Shadow treasurer Joe Hockey yesterday said the Coalition would not finalise its costings until after the release of the Pre-Election Economic and Fiscal Outlook, which is due to happen after the writs are issued in August.

It prompted a swift response from the Government, which demanded the Coalition "come clean" on what it is planning.

"This is a deeply deceptive ruse from the Coalition to hide the impact of their policy proposals from the Australian people," Treasurer Wayne Swan and Assistant Treasurer David Bradbury said in a joint statement.

"The idea that the Coalition can refuse to release what any of their policies cost until deep into the campaign is unprecedented.

"There is nothing stopping the Coalition releasing today the policies they boasted to have already been costed by the Parliamentary Budget Office."

But Mr Hockey hit back, saying he will not "cop a lecture" from Labor over policy postings.

"What we have identified correctly is the only numbers that we can rely on are the Pre-Election Fiscal Outlook numbers released by the heads of department during the course of the election campaign," he told reporters in Canberra.

"They are the same numbers that the Labor Party and ourselves relied on before the last election.

"We are not going to cop any lecture from Labor about numbers. They've got every single number wrong for the past five years."

The Coalition has attacked Labor for walking away from its commitment to deliver a budget surplus this financial year.