Justin Field new NSW Greens upper house MP

An anti-coal seam gas campaigner who helped establish the Lock the Gate movement will become the next Greens politician to join the NSW Legislative Council.

Justin Field won a vote of party members on Saturday to fill an upper house vacancy left by the death of Greens MLC John Kaye, after a pre-selection campaign which exposed factional tensions.

"Preselections in any party are tough but I'm humbled by the strong support of the membership. I know they want us to get on with the job," Mr Field said after claiming victory.

"I'm looking forward to joining my Greens colleagues in the parliament and working with members across the state to continue the work building a more sustainable and fair society."