There is one "overriding" reason John Howard would never have stepped aside in 2006, says Peter Costello - the then prime minister and his wife, Janette, were hell-bent on hosting the APEC summit in September 2007.

Several of Mr Costello's former cabinet colleagues now claim Mr Howard would have left at the end of 2006 had the "walletgate" scandal not erupted that July.

Mr Costello, who says Mr Howard was always finding reasons to stay, is dismissive of this claim.

He says, "By that stage, APEC was well advanced.

"The program was prepared, the entertainment and the outfits (always a big thing at APEC meetings) had been chosen.

"Mrs Howard was involved.

"The summit was to take place in Howard's home town, Sydney, where he had established his official residence at Kirribilli House.

"Neither John nor Janette Howard would want this high-level summit to be hosted by anyone else in their city at their Kirribilli residence," he writes acidly in his memoirs.