Prime Minister John Key is defending new legislation that prevents people taking legal action against the Government, saying it is constitutional and necessary.

A bill legislating for the payment of family members looking after disabled adults was adopted by Parliament under urgency at the weekend.

It followed a Court of Appeal ruling against the Ministry of Health policy excluding family members from payment.

Critics say a clause in the bill preventing family members taking further legal action is unconstitutional.

Concerns have also been raised about the bill's regulatory impact statement, much of which is blacked out because it is "legally privileged".

Green Party MP Catherine Delahunty said Parliament had failed the families and the bill perpetuated existing discriminations.

But Mr Key believed the clause was legal, constitutional and necessary. It highlighted "a really challenging issue about where you draw the line between family responsibility and where the state should play a role".

About 1600 high-needs disabled adults will be able to pay family members to care for them. It will cost the Government $92 million over four years.