Felicity Wilson, who was backed by the party's left faction, won a preselection ballot on Wednesday night 104 - 98 to defeat the right's candidate Tim James, a former chief of staff to planning minister Anthony Roberts.

Ms Wilson is a former deputy executive director of the NSW division of the Property Council of Australia and previously worked in corporate affairs for energy company Caltex. She is a former president of the NSW Liberal Party's women's council.

The ballot was decided just before midnight after a marathon five rounds of voting.

Ms Wilson will take on a growing list of independents at the April 8 byelection in which the issue of forced council amalgamations is expected to be centre stage.

North Sydney mayor Jilly Gibson announced on Wednesday she would contest the byelection, less than a month after she back-flipped on her opposition to North Sydney council's anti-merger campaign, throwing her support behind the council's legal case against the NSW government.

It marked the first time she had supported the council's legal fight since the proposed merger with Mosman and Willoughby councils was announced in May 2016.

The anti-merger sentiment is expected to hurt the government at the April poll, but the crowded independent field will likely split the backlash vote.

Other independent candidates include Royal North Shore Hospital surgeon Stephen Ruff, Mosman councillor Carolyn Corrigan, and lawyer Ian Mutton.

On Monday the Liberals preselected James Griffin to contest the April 8 byelection in former Premier Mike Baird's old seat of Manly.

Mr Griffin, a former deputy mayor of Manly, is an executive with professional services group KPMG.

He easily defeated businessman Walter Villatora, who was backed by former prime minister Tony Abbott, by 71 votes to 37.

A byelection in the seat of Gosford will also be held on April 8 after the retirement of Labor's Kathy Smith due to ill health.

Labor's candidate is former Paralympian Liesl Tesch and the Liberals have preselected Jilly Pilon.