The NSW Greens membership base has spread its support across the party's rival factions, in a bitter preselection battle which pitted the left and right flanks against one another.

Cate Faehrmann, who previously served on the Legislative Council from 2011-2013, will return to NSW politics after being preselected to replace Dr Mehreen Faruqi, who is poised to transfer to the Senate.

The preselections, which are determined by the party's grassroots memberships, were held on Saturday to fill both the casual vacancy and to select the party's NSW upper house ticket for March 2019 election.

David Shoebridge claimed the top spot on the party's ticket ahead of his rival Jeremy Buckingham, securing 1161 primary votes compared with Mr Buckingham's 780 votes.

Mr Shoebridge's victory is a critical win for the party's left faction, after Senator Lee Rhiannon lost the top Senate ticket spot to Dr Faruqi last year.