Green Party co-leader James Shaw says the party will lead by example on gender equality by ensuring half of its Cabinet members in a future government are female.

The Green Party will ensure half of its Cabinet ministers are women if it becomes part of the next Government, co-leader James Shaw says.

Speaking at the Council of Trade Unions conference in Wellington, Shaw said the Greens would also ask any other parties in a coalition government it was involved in to make the same commitment as part of a drive for gender equality.

Shaw said the party wanted to lead by example and improve the gender balance in Parliament, with just 30 per cent of Cabinet positions currently held by women and only 32 per cent of current MPs female.

"If political parties are serious about ensuring women across all workplaces are paid more and given the opportunity to take on senior roles, then political parties should start by getting their own houses in order."

He said there were at least 28 other countries which had a greater proportion of women in Cabinet, including France, Canada, Germany, Israel and South Africa.

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Shaw said the commitment was important given news of a growing gender pay gap, with men paid nearly 12 per cent more an hour than women in median hourly pay this year.

He moved to head off criticism of the policy on the grounds that hiring should be based on merit, calling it "a virtuous aspiration that usually causes more harm than good" and led to men being chosen over equally or more talented women.

"Take a look at our current Parliament which is seventy percent male. Or Cabinet, which governs the country, also seventy percent male.

No one seriously thinks all those guys are there because they're the best of the best, or that they've all got so much more merit than any female politicians...the reality is that it's a traditionally male institution."

The Green Party already ensures that it has both a male and female co-leader, while it has an even balance of men and women on its party list for each election.