Prime Minister Scott Morrison has confirmed Malcolm Turnbull knocked back approaches from Dave Sharma seeking his support during the Wentworth by-election.
Mr Morrison says the ousted prime minister turned down a number of offers from Mr Sharma and senior Liberal Party figures to get behind the Liberal Party's candidate.
Independent candidate Kerryn Phelps is on track for a historic win in Wentworth after achieving an unprecedented swing of more than 20 per cent.
A loss for the Liberal Party would place the coalition into a minority government with just 75 seats in the House of Representatives, while the Labor party has 69 lower house MPs.
But Mr Morrison is unsure how much of a difference a strong recommendation from Mr Turnbull would have made to the campaign.
"Quite a number of us asked for that support, not necessarily in the form of a letter, there are many other ways in which people can choose to express their support," Mr Morrison told reporters on Sunday.
"There were even approaches made by Dave himself. What impact they would have had, ultimately, is for others to judge.
Mr Turnbull was the Member for Wentworth from 2004 until quitting federal parliament in August after being replaced as prime minister.
He has been holidaying in the United States after his resignation sparked the by-election.
Australian Associated Press
