The Liberal Party will pre-select its candidate for the blue ribbon seat of Wentworth on Thursday evening, only days after the male frontrunner pulled out to make way for a woman.

It is understood Andrew Bragg, who was considered favourite to succeed Malcolm Turnbull in harbour-side seat, pulled out under mounting pressure from Canberra and in exchange for a safe Senate spot.

Scores of party faithful filed into the Eastern Suburbs Rugby Union Club in Rose Bay on Thursday to decide who will contest a by-election, which some polls indicate could come down to the wire.

Former Liberal staffer and Woollahra deputy mayor Katherine O'Regan has emerged as the favourite to be pre-selected in Mr Bragg's absence.

It is understood she has the support of Prime Minister Scott Morrison and other high ranking Canberra power brokers.

Former Australian ambassador to Israel Dave Sharma is also in the mix, but has also been pressured by Canberra to pull out and make way for a woman candidate.

However, Mr Sharma is understood to have garnered support from Mr Turnbull and former prime minister John Howard, Fairfax reports.

Mr Turnbull reportedly called Mr Sharma from New York, urging him to stay in the race amid mounting pressure to bail out.

Woollahra councillor Mary-Lou Jarvis is also expected to receive more votes than others in a field that includes former Wentworth member Peter King and campaigns director Carrington Brigham.

There are eight candidates in total.

Independent candidate for Wentworth, Licia Heath, says some voters would be skeptical of the true reasons behind the Liberals promoting a woman in the seat.

"(Voters) are saying 'they weren't going to do it five days ago, now they are because of the numbers, not because they suddenly have a belief in all the benefits that a woman could bring'," Ms Heath told AAP.

Mr Morrison said pre-selectors had a very important job, given the "very difficult" circumstances around the by-election.

"I think this is going to be a very close fight," he told reporters on Thursday.

"The former prime minister had no doubts about the difficulty of contests in Longman, Braddon and Mayo and I similarly have no expectations about results (in Wentworth)."

The by-election will be held on October 20.

There are fears within the Liberal party a high profile independent could shake things up, with same sex marriage campaigner and City of Sydney councillor Kerryn Phelps still considering a tilt.

Labor has selected businessman and Tamarama Surf Lifesaving president Tim Murray to run in the seat, which has never been held by any party other than the Liberals.

Australian Associated Press