A minority federal parliament could be on the cards, according to the latest polling in the Wentworth by-election.
The Voter Choice research, which sampled 736 voters in the Sydney seat, puts independent Kerryn Phelps on 55.4 per cent of the two-candidate preferred vote, ahead of the Liberals' Dave Sharma.
The Liberals need to retain the seat, previously held by Malcolm Turnbull, at the October 20 by-election to hold onto their one-seat majority in parliament.
In primary terms, Mr Sharma is sitting on 38.8 per cent, with Dr Phelps on 23.5 per cent and Labor's Tim Murray polling 17.2 per cent. The poll has a three point margin of error.
If Dr Phelps, a popular health commentator and local doctor, can get enough preferences from Labor, the Greens and other independent candidates she will have a strong chance of winning.
So far more than 5000 Wentworth voters have cast early votes with 11,000 applying for a postal vote.
The polling came as Mr Turnbull's son urged Wentworth voters not to back the Liberal Party.
Prime Minister Scott Morrison said he disagreed with Alex Turnbull.
"His father disagrees with him too," Mr Morrison told the ABC on Friday.
"His father Malcolm Turnbull is heavily supporting Dave Sharma, the only Liberal candidate running for Wentworth."
Mr Morrison says the former prime minister has recently been silent on the issue as he has bowed out of public life.
Alex Turnbull has posted a video online telling voters in Wentworth to vote against his father's party, in part due to concerns about the party's lack of action on climate change.
"We're going to have an election within 12 months anyway, so if you want to send a signal as to which way the Liberal Party is going, and your displeasure with where it's going, then this is your opportunity," Alex says in the video.
"Don't vote for the Liberal Party in the Wentworth by-election.
"If you want to pull the Liberal Party back from the brink it's the one clear signal you can send."
Dr Phelps likened the Liberal Party to a sick patient "heading down a disastrous path".
"Alex Turnbull is absolutely correct in saying that voters should vote for someone other than the Liberal Party in the Wentworth by-election and consider it an intervention," Dr Phelps said.
Defence Minister Christopher Pyne says Alex Turnbull is campaigning for instability, as the coalition will lose its one seat majority in the House of Representatives if defeated in Wentworth.
"I know Alex, he's a great bloke, but unfortunately on this occasion he has chosen to campaign against the Liberal party," Mr Pyne told Channel Nine on Friday.
"Which means a campaign for instability and we don't need instability right now, we need stability."
Malcolm Turnbull resigned from his seat after losing the Liberal leadership at the end of August, sparking the by-election.