Prime Minister Tony Abbott took less than 24 hours to agree to re-examine the Government's policy on the $20 billion future submarine project, in an effort to shore up votes against a leadership spill last month.

The ABC's Four Corners program can reveal that Cabinet's top secret National Security Committee (NSC) met in October last year and supported a move that would allow the bulk of Australia's submarine fleet to be built offshore.

The sensitive decision was not announced at the time, although a press release had been drawn up for then defence minister David Johnston.

In February, the weekend before the spill motion, South Australian senator Sean Edwards told Mr Abbott his vote would depend on whether local shipbuilders, including the Australian Submarine Corporation, would be given the opportunity to participate in a tender for the contract.

"He rang me at 6.30 on Saturday night and I heard from him at ten past three the following Sunday, the next day," Senator Edwards told Four Corners.

"He said he'd had a discussion with the defence minister and they'd come to a position on this, which obviously I was seeking."

Policy change not discussed with Bishop

Mr Abbott did not raise the submarine discussions that weekend with Foreign Minister Julie Bishop, a spokeswoman for Ms Bishop told the program.

This was despite Ms Bishop being a member of the NSC.

Mr Abbott's agreement with Senator Edwards again revisited the previous outcome of the October NSC meeting, which had broken an election promise to construct the submarines in South Australia.

Details of the highly confidential October NSC meeting were based on accounts from sources closely involved in the submarine project.

Despite press speculation at the time, the NSC did not make any final decision to build the submarines in Japan.

However, it did decide to open the way for their construction overseas because of time and cost constraints.

South Australia's Minister for Defence Industries, Martin Hamilton-Smith, told the program his State Government "kept receiving feedback ... that the Japan option was very much the option".

"In fact, I was told we may as well give up," Mr Hamilton-Smith said.

Following the leadership spill, Mr Abbott reconvened the NSC and Defence Minister Kevin Andrews announced on February 20 that the Future Submarines Project would involve a "competitive evaluation process".

Under that process, the Minister said the Defence Department would seek proposals from partners that included "options for design and build" of the submarines "overseas, in Australia and/or a hybrid approach".