A former military intelligence officer will become the next Greens politician to join the NSW legislative council, after the preselection of Justin Field by party members on Saturday.

Mr Field, a campaigner for the Nature Conservation Council who also helped establish the Lock the Gate movement to oppose coal seam gas, fills the upper house vacancy left by the death of Greens MLC John Kaye.

The new Greens NSW upper house MP Justin Field.

He was backed by Lock the Gate president Drew Hutton, who posted a video online in support of Mr Field's "strong sense of strategy".

The win appears a triumph of the environmental movement and social media-savvy campaigners over an old guard in the NSW Greens with strong union links.