Judith Collins says the Green Party has sold any ability to oppose anything under its cooperation agreement with Labour.
The deal was signed on Sunday morning after a majority of Green members voted in favour of it, landing co-leaders James Shaw and Marama Davidson with some ministerial roles outside of Cabinet.
National leader Collins told Magic Talk's Peters Williams on Monday the agreement is "just a whole spin on that nonsense" of voting Labour to keep the Greens out of power.
There was speculation before the election that traditional National voters swung to Labour so it would get a majority - which it did - so the party wouldn't need the Greens to form a Government and compromise with its policies, many of which are further left.
"I said all the way through [the election campaign], 'you do that [tactically vote Labour] they'll just bring them in any way', and so they have," Collins said.
"They don't need the Greens there - that's why the Greens have been very happy to sell their objection to almost anything and their opposition to the Government and to Labour, on the basis of two ministerial positions outside Cabinet. It's a very cheap deal in terms of what the Greens get for what they give up which is any sort of oppositional ability against Labour."
Davidson and Shaw were appointed Climate Change and Family and Sexual Violence Prevention Ministers - both roles outside of Cabinet.
"We are really delighted that we do have a win-win agreement here," Shaw said on Sunday.
But Collins said the Greens have been given "two portfolios they can never win on".
And her comments echo those of even former Green MPs - who say the party will become "gagged" under the agreement.
"They will simply be an ineffectual lapdog of Labour for the next three years," former Green MP Sue Bradford said.
"Once they're inside Government they're effectively gagged, so they can't set the agenda on many issues but particularly core issues like climate change," said Russell Normal, the Greens' former co-leader.
Labour didn't need the Greens to form a Government, obtaining a clear majority of 64 seats at the election - three more than it needed.