One-time NSW Greens MP Jeremy Buckingham has taken a swipe at his most-likely replacement on the upper house ticket, corporate lawyer Abigail Boyd, in his most substantial intervention in the affairs of his former party since quitting.

Mr Buckingham said it was "astonishing and disappointing" that Ms Boyd was a leading Greens candidate in the March election, suggesting her past work at corporate law firm Allen & Overy — which frequently works for major energy firms — was at odds with the party's platform.

Fed up: Jeremy Buckingham quit the Greens last December.

But Ms Boyd hit back, and said environmental issues "are not niche issues", and the party would benefit from a candidate with her extensive experience in finance and economics.

Despite early voting for the state election beginning in three weeks, the NSW Greens still face a stoush over their upper house ticket, with a long-simmering conflict between Ms Boyd and Mr Buckingham's party ally Dawn Walker, now in Parliament, unresolved.