PM responds to Turnbull's Labor campaign

Malcolm Turnbull's son is soliciting donations for the Labor Party in the Sydney by-election to replace his father, but Prime Minister Scott Morrison says it's all part of democracy.

Alex Turnbull has taken to Twitter to encourage people to donate to the Labor Party ahead of the Wentworth by-election, saying it's the best "bang for the buck" they'll get.

He is also sharing campaign material from Labor's candidate Tim Murray via social media.

Asked on Sunday if it struck him as odd the son of a retiring Liberal MP was campaigning for Labor, Mr Morrison turned the other cheek saying: "it strikes me as a democracy".

Responding to online criticism over his new found support for Labor, Alex said: "If you want blind unthinking faith you can go to a place of worship".

"If all you are is barracking for a football team in politics you're just a useful idiot foot soldier for the vested interests that run the place. Wake up."

The Liberals are facing a tough fight in Wentworth, with a recent poll placing them even with Labor on a two party preferred basis.

There is concern within the party that a strong independent candidate could also do some damage, with local GP and marriage equality campaigner Kerryn Phelps likely to throw her hat in the ring.

While Mr Turnbull's resignation has prompted several high profile names to push for Liberal pre-selection, AAP understands his close friend and former Business Council of Australia executive director Andrew Bragg is shaping as favourite.

Former Australian ambassador to Israel Dave Sharma also in the gunning for the Liberal ticket, but senior moderates are pushing for Mr Bragg to be the consensus candidate.

The former prime minister and his wife Lucy on Sunday flew out of Australia for a holiday in New York.

Mr Turnbull formally resigned from parliament last Friday, a week after Mr Morrison defeated Peter Dutton in a leadership spill.