The Prime Minister is defending the next step in deciding Ports of Auckland's future

A working group's concluded Northport is the preferred option for major freight operations - and Cabinet agrees things can't carry on in Auckland.

But a plan won't be made until May.

Jacinda Ardern says more analysis is needed.

She says it's an enormous decision about a significant piece of infrastructure, key to imports and exports.

Among things to be considered are the road and rail investments needed to make the move a reality.