The Greens have drafted a bill to overhaul adoption laws and allow adoption by same-sex couples, saying legislation about to be debated by Parliament doesn't guarantee New Zealand's antiquated legislation will change.

The member's bill by Kevin Hague would enable gay adoption, something already being considered in two Labour bills already before Parliament by MPs Jacinda Ardern and Louisa Wall.

Hague's bill is the result of cross party work with National MP Nikki Kaye. Labour was part of that work until MP Jacinda Ardern pulled out of the group last year and drafted her own bill to overhaul adoption laws.

While Hague's bill has to be drawn from the member's ballot before it is debated by Parliament, Ardern's bill is expected to have its first reading within the next month.

Hague today said the Greens would not support Ardern's bill because it required the Law Commission, which reviewed the 1955 Care of Children Act in 2000, to draft a bill and then for the Minister of Social Development at the time to introduce that legislation.

Even in the best case scenario, under Ardern's bill it would be two or three years before a law was drafted, he said.

"I don't think the process adds any advantage. It's sole advantage is that it is already on the order paper. We already have a bill."

The Greens had been told by Parliamentary officials that Ardern's bill could not be substantially amended to include the cross party work and speed up the process, Hague said.

The controversial issue of gay adoption will be addressed in Wall's bill which legalises gay marriage, therefore enabling gay adoption by defining same sex married couples as spouses. Wall's legislation is a conscience bill which has already passed its first reading.

Ardern's bill is expected to fail its upcoming first reading without the support of the Greens.

Hague said his bill would modernise adoption in New Zealand so all adoptions are open unless there are exceptional circumstances.

It also removes restrictions on the kinds of people who can adopt so adoptive parents are selected in the best interests of the child. It enables the Maori cultural practise of whangai, where children are raised by other members of the whanau, and enables children born through surrogacy to be adopted.

- © Fairfax NZ News