The Government is keeping its options open, as it prepares for the eventual replacement of the Air Force's six P-3 Orions.

New Zealand has been given approval to buy four P-8A Poseidon maritime patrol planes from the US, at a cost of almost $1.5 billion.

Defence Minister Gerry Brownlee says approval is only a formality, and a decision is still a long way off.

Mr Brownlee said the P-8As will be seriously considered because of their capabilities, but it's one of many options.

"It's just one of a number of aircrafts that the Ministry of Defence procurement team will be looking at alongside the Defence Force as a possible replacement for the P-3 Orions," he said.

Mr Brownlee also said that a rescue mission in Tonga this week showed the P-3 Orions still have years left in them.