National says the Greens have sold their soul to please Winston Peters.

The Greens voted in favour of the "waka jumping bill" last night during its second reading.

Green MP Golriz Ghahraman says it was a difficult choice, but they're bound by good faith to support it, because New Zealand First wants it.

"Deciding to be part of this new Government is the biggest opportunity in our party's history to make great green change. We intend to take that opportunity."

But Electoral Law spokesperson Dr Nick Smith says Green co-leader James Shaw had raved that the advantage of their confidence and supply agreement with Labour was this.

"Green MPs will not vote for anything they do not agree with it - that is exactly what is happening here."

Smith says the Greens have stooped to a new low.

"Co-leader Marama Davidson, she says it is a threat to democracy, but they are voting for it."

The bill being, promoted by New Zealand First, prevents an MP from leaving a party but staying on in Parliament as an independent.

It also allows MPs to be kicked out of if three quarters of their caucus wants them gone.