An artist's impression of what an extended Wellington Airport runway would look like.

The pilots' union has lost its bid for longer runway safety areas at Wellington Airport, clearing a major hurdle standing in the way of a proposed $300 million extension.

A case taken by the New Zealand Air Line Pilots' Association to the High Court in Wellington was rejected this week by Justice Karen Clark.

Wellington Airport wants to extend its runway south by 355 metres to allow for long-haul flights from Asia and the United States.

ROSS GIBLIN/FAIRFAX MEDIA A plane lands from the southern approach to the existing airport runway.

But in November 2015, the association argued the extension plans should include 240-metre safety zones rather than the 90m ones approved by the Civil Aviation Authority (CAA).

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Safety zones reduce risk for planes under or over-shooting the runway.

The pilots' association argued the CAA decision was flawed because it was wrong in law, and it did not properly consult with the association. It wanted the CAA to reconsider its decision.

The CAA argued the including 240m safety zones would double the $300m price tag of the extension.

In her written findings, Justice Clark ruled the CAA did not breach its duty to consult, and did not err in law. She dismissed the association's application.

Airport chief executive Steve Sanderson said the airport was pleased the judicial review was dismissed so "emphatically".

The airport already operated with 90m safety areas, and airlines would not fly there if they did not feel it was safe, he said.

It was "hypothetical" to say whether the project might have been stopped if 240m safety zones were required.

"A judicial review is about process: had they consulted? Had they followed the civil aviation rules? In our view that was quite clear, and Justice Clark agreed."

Pilots' association senior technical officer David Reynolds said the organisation would review the decision before commenting on what to do next, which could include an appeal.

Richard Randerson, spokesman for anti-extension group Guardians of the Bays, said if the pilots had successfully required a longer safety zone it would "probably have put an end to the whole project".

"While the pilots' case might have been helpful in heading the whole thing off, the response from us will continue and we have it all well in hand."

The decision comes after a resource consent application for the extension was filed in April. Public submissions are open until August 12.

Wellington Deputy Mayor Justin Lester said the High Court decision was another "step in the right direction" for the extension project.

CAN PILOTS JUST SAY NO?

Aviation expert Irene King said pilots had the right to refuse to land at any airport, but it was unlikely they would do it at Wellington simply because of its 90m safety areas.

"I would be very surprised. The CAA's decision on this is entirely within the rules. The pilots would actually be saying 'we actually know better'. That's a big call."

Runway safety areas were useful for "when shit really happens" in cases such as a plane aquaplaning across the ground, King said.

But New Zealand pilots were trained to avoid such ground conditions in the first place, she said.