Tony Abbott.

Tony Abbott. Photo: Andrew Meares

The Coalition plans to start axing federal public service jobs just three weeks after it is elected.

A Tony Abbott-led government will then need to cut more than 660 jobs a month between October and June next year as it looks to move quickly to lock in its projected savings.

Correspondence between the Parliamentary Budget Office and senior Coalition frontbenchers shows that the axe will fall on public service jobs in just over a month if Mr Abbott wins government.

The documents, which have been used to underpin Coalition costings, confirm the target of 6000 positions to be axed between October 1, 2013, and June 30, 2014, with the opposition still insisting that the cuts will be achieved through natural attrition.

The Coalition believes its first tranche of cuts will be enough to reach its ambitious target of saving $303 million this financial year if it forms government after the September 7 election.

Fairfax Media understands there are plans for another 6000 jobs to be cut in the 15 months between July, 2014, and September, 2015, and savings of $1.2 billion in the 2014-2015 financial year are expected, according to the correspondence between shadow ministers and the PBO.

The Coalition's plan, which has still not been made public in detail, then calls for $1.7 billion in savings in 2015-2016 and $1.92 billion in 2016-2017 - $5.1 billion ripped out of the federal public service in the forward-estimates period.

The revelations come as the two main political parties continued to exchange blows over public service job cuts, with Finance Minister Penny Wong releasing Parliamentary Budget Office data she said proved the Coalition would have to cut 20,000 public servants to achieve its saving targets.

Coalition campaign spokesman Christopher Pyne hit back, demanding to know why the costings had been commissioned in March this year and accusing the government of planning its own massive cuts.

"Kevin Rudd and Penny Wong must come clean and explain why the Labor Party asked the independent Parliamentary Budget Office to cost a 20,000 reduction in public servants," Mr Pyne said.

Despite the Coalition's need to cut more than 160 jobs a week in its first nine months in office, Mr Pyne insisted that there would be no forced redundancies.

"There is no ambiguity about the Coalition's position," he said. "We have clearly stated that, if elected, we will reduce the Commonwealth public service by 12,000 through natural attrition.

"Independent fact-checking units have found Labor's claims that an incoming Coalition government would cut 20,000 jobs to be false."

The Community and Public Sector Union also weighed in on Wednesday, saying Senator Wong's figure showed the Coalition had a "credibility problem'' on public service jobs.

"The information released today by the Liberal Party does nothing to address their growing credibility problem on public service cuts," national secretary Nadine Flood said.

"One minute the Liberals are claiming the public service has grown by 20,000 under Labor. The next they want us to believe Labor have cut 10,000 public service jobs. They can't have it both ways."

The union also said the opposition's position on natural attrition was contradicted by its own Parliamentary Budget Office costings.

"The PBO paper released today poured cold water on the Coalition's claim it will save $5.2 billion by reducing the public service by 12,000 through natural attrition," Ms Flood said.