Malcolm Turnbull has defended his son, who’s supported people donating to the Labor Party campaign for the by-election for his father’s seat of Wentworth.

Alex Turnbull tweeted on the weekend that donations to the Labor candidate were “the best bang for the buck you’ll get”.

Arriving in New York for a holiday, with wife, Lucy, the former prime minister said Alex is entitled to his views.

“My son is 36 years old,” Mr Turnbull said.

“He’s entitled to his own political opinions.”

Malcolm Turnbull said Alex is no longer the son of a prime minister and is free to express himself.

Mr Turnbull formally resigned from parliament last Friday, with a by-election in Wentworth expected next month.