Barnaby Joyce has slammed former prime minister Malcolm Turnbull’s decision to quit parliament and trigger a by-election in his prized Sydney seat of Wentworth.

Mr Turnbull told supporters at an electorate conference on Monday night he would resign on Friday.

Mr Joyce, the former deputy prime minister, said it would have been more honourable if Mr Turnbull saw out his term from the back bench.

“You leave on the right terms with the electorate and the right terms with your nation,” Mr Joyce told the Seven Network on Tuesday morning.

“To say ‘oh well I’m not the prime minister anymore, ho hum, you guys might lose government’ – the government that he led – I think people are really disappointed with him about that,” he said.

The next federal election is not due to be held until mid 2019, but Mr Turnbull’s decision sets up a dangerous contest to replace him.

Without Mr Turnbull, Parliament will return on September 10 with the Coalition deprived of its one-seat majority.

Winning a Labor-induced vote of no confidence could prove difficult – although not impossible if the crossbench co-operates.

Mr Turnbull easily won Wentworth with a margin of 15 per cent or more in 2010, 2013 and 2016. But the margins were previously much slimmer: just 3.85 per cent in 2007 and 2.51 per cent in 2004.

Mr Joyce and Mr Turnbull fell out spectacularly in February amid a storm of controversy surrounding the then Nationals leader’s affair with a staffer.

Mr Turnbull has held the seat of Wentworth for 14 years and said last week he would not stay on the back bench if he was deposed as leader.

Jostling to fill the harbourside seat has begun, with Liberal backbencher Tony Abbott endorsing his sister for the plum seat just days after he helped orchestrate Mr Turnbull’s demise.

Christine Forster confirmed on Monday she will run for Liberal pre-selection.

“Do two Abbotts make a Bishop? I don’t know,” Mr Joyce quipped.

Dave Sharma, Australia’s former ambassador to Israel, is considered another frontrunner to win preselection, along with Andrew Bragg, who ran the Liberals’ ‘Yes’ campaign in the marriage equality plebiscite.

Mr Abbott said he believes his sister would be a “wonderful competitor” in the looming by-election.

“If she does put her hand up for Wentworth and if she is successful, I know she will be a wonderful, wonderful competitor in any by-election and she’d be a great local member if she were elected,” Mr Abbott told reporters in Sydney.

Ms Forster, a City of Sydney councillor, said she had a strong record of championing Liberal Party values at a local government level.

“I think as someone with local credentials and a strong track record of strong liberalism, I feel they are the types of candidates we want in any seat,” she told AAP.

Opposition Leader Bill Shorten acknowledged Wentworth would be “a hard seat for Labor to win”.

“It’s a margin of nearly 18 per cent, but we’re still considering our options there,” he told ABC radio on Tuesday.

Labor’s candidate for the seat, local businessman Tim Murray, has earned praise from a potentially surprising source – Mr Turnbull’s Singapore-based son.

Alex Turnbull said it is “hard to back a Labor guy but not Tim”.

“He’s a great guy and I know him well,” the fund manager told Fairfax Media.

Independent Tasmanian MP Andrew Wilkie believes the right candidate will have a chance of snatching the seat from the Liberals.

“It’s not a sure thing that the Liberals will retain Wentworth,” he told ABC Radio on Tuesday.

“It is ripe for a really high-profile independent to pick up the disaffected Liberal voters and also the more progressive voters.”

NSW independent MP Alex Greenwich confirmed on Tuesday he would not run as an independent in the seat, while GP Kerryn Phelps is still considered a tilt.

-with AAP