New South Wales Liberal Party acting president Trent Zimmerman has been chosen to contest Joe Hockey's former seat of North Sydney in a by-election in December.

Mr Zimmerman beat two rivals in a pre-selection vote held in Chatswood on Monday night.

Restaurant and Catering Australia chief executive John Hart and Defence official Helen Cartledge also contested the ballot.

Mr Zimmerman, a former staffer of Mr Hockey, won with an outright majority, claiming 47 of a possible 84 votes.

His preselection is expected to be officially endorsed by the party's executive on Tuesday.

The December 5 by-election has been forced by the retirement of former treasurer Joe Hockey.

Process described as undemocratic 'stitch-up'

The process to appoint the new federal Liberal candidate has been described from those within the party as a "stitch-up" and undemocratic.

With a 15.9 per cent margin, North Sydney is one of the safest federal Liberal electorates in Australia.

The seat was held by Mr Hockey for nearly 20 years and now the party was facing a once-in-a-generation opportunity to choose a new candidate.

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But some insiders said that opportunity had been missed, and instead of a genuine competition the safe seat was being effectively handed to a party apparatchik.

Barrister and Liberal Party member Juris Laucis was disappointed in the process, saying that the Liberal Party machine had guaranteed that Mr Zimmerman would win the contest.

"Trent Zimmerman has almost 50 per cent in the bag even before you start the pre-selection voting, and that's not democratic," he told 7.30, describing the pre-selection process as a "stitch-up".

The candidate was decided by a vote of 48 members from the North Sydney branch, and around 40 from the Liberal Party's state executive and state council.

There would have been 48 from the central branch, but 7.30 understands eight of them had been ruled out on technicalities, such as not attending a sufficient number of Liberal Party meetings.

Mr Laucis, a member in the neighbouring Warringah branch, said a decision by the NSW Liberal Party not to allow all the branch members of North Sydney to vote for a candidate was a mistake.

Protesters outside the Liberal Party pre-selection meeting in North Sydney. ( ABC News: Jean Kennedy )

He risked expulsion from the party by speaking out about the pre-selection process.

"I'm fully aware that as member of Liberal Party I am not supposed to speak about party matters to the press, however when there is no avenue of complaint within the constitution when you feel the executive are not doing the right thing there is no other choice," he said.

"If it means I get expelled so be it, but it's not really worth being part of a party that does not practice democracy."

North Sydney has been represented by an independent in the past. The seat was held between 1990 to 1996 by Ted Mack.

The Australian Electoral Commission estimates that the by-election will cost over $1 million.