Malcolm Turnbull's son solicits Labor donations

Malcolm Turnbull's son is encouraging people to donate to Labor ahead of the Wentworth by-election for the ex-prime minister's former seat.

Alex Turnbull has re-tweeted Labor candidate Tim Murray's call for donations for the upcoming poll for the federal Sydney electorate.

"Best bang for the buck you'll get in political donations in your life," the former Liberal leader's son said in his re-tweet of Mr Murray's post on Saturday.

"Tight race, tight margin for the government, big incremental effect whatever happens.

"If you want a federal election now this is the means by which to achieve it."

Alex Turnbull has been outspoken online and in the media since his father stepped down as Liberal leader following a challenge from cabinet minister Peter Dutton.

Malcolm Turnbull earlier said it was best former prime ministers did not stay in parliament and formally resigned on Friday, triggering a by-election in the inner-Sydney seat of Wentworth.

He entered federal parliament in 2004 and became prime minister in 2015 after rolling then-leader Tony Abbott.

Running for Liberal pre-selection for Wentworth are former Business Council of Australia executive director Andrew Bragg, Mr Abbott's sister Christine Forster and ex-Australian ambassador to Israel, Dave Sharma.