Same sex couples would be able to adopt under proposed changes to New Zealand's 50-year-old adoption law to be debated by Parliament.

Labour MP Jacinda Ardern's Care of Children Law Reform Bill was today chosen from the member's ballot.

Ardern said her bill was based on recommendations made by the Law Commission 10 years ago.

At present only married couples, single individuals - gay or straight - and de facto couples could adopt, she said. Civil Union couples cannot.

Parliament is already considering a bill by fellow Labour MP Louisa Wall to legalise gay marriage.

"What Louisa's bill does is enable gay couples to adopt because they would be considered spouses. What mine does is make it more fit for purpose for all families who are using it.

"At the moment our adoption laws are based on closed adoptions. The people on the adoption order are the only people who maintain a relationship with that child. So the basic idea is to change the way we make decisions so the child is at the centre of everything and so it allows us to make decisions in their best interests."

Ardern's bill was one of four bills chosen today.

New Zealanders would be able to swear allegiance to the Treaty of Waitangi under Maori Party MP Te Ururoa Flavell's Oaths and Declarations (Upholding the Treaty of Waitangi) Amendment Bill.

The other two bills were Labour MP Darien Fenton's Local Government (Public Libraries) Amendment Bill and National MP Scott Simpson's Land Transport (Admissibility of Evidential Breath Tests) Amendment Bill.

It is the second bill for Fenton to be chosen from the ballot in a fortnight after her bill to make ratepayer-owned ports subject to the Official Information Act was picked earlier this month.

Her new bill would make all local authorities give libraries enough funding to ensure there was no charge for books and digital information. Some libraries now charge for the hire of DVDs and CDs.

Simpson's bill would close a "small but significant loop hole" in breath testing laws which meant breath tests were not admissible in court if people elected a subsequent blood test, but blood was unable to be taken.

People who blow tests over the legal limit have the right to request a blood sample.

"In the case of an intravenous drug user, if there was difficulty finding a vein, and if for some reason a blood sample was not able to be taken it would be good to be able to refer to the breath test."

Two legal judgments had highlighted the loophole and Simpson said it was important such cases weren't struck out of court.