Some long-haul flights to China and Los Angeles could only take off with hundreds of empty seats on Wellington's proposed runway extension, according to a new report.

The performance review of the AU$300 million runway, commissioned by the airport, says five out of nine big jets it looked at still could not safely take off with a full passenger load.

Wellington Airport has consistently said a 355m runway extension would allow Boeing 787s and Airbus A350s to give Wellington a direct link to Asia and North America.

But one of the planes, the Boeing 787-8, could not use the runway at all for long-haul flights to Singapore, Guangzhou, Beijing and LA unless it is practically empty.

That is because most of those aircraft did not have powerful enough engines and would need to be upgraded with new software.

Even then, flights to Beijing would have 100 empty seats on a wet, northerly day, and LA would have 50.

Only one plane, a brand new Airbus A350-900, can make it to China's capital at full capacity, but the runway must be dry.

Runway opponent John Beckett who represents airlines that fly to New Zealand, said most carriers would not consider a route if they could not take a full load of passengers.

"A number of the routes put forward by the airport are doubtful in terms of the ability to carry full passenger loads for the necessary distance," he said. "Without the ability to carry full passenger loads the economic viability of these routes is put severely into question."

Wellington Airport has pointed to Asia -- particularly China -- as a future growth market.

But Mr Beckett said if the flights could not make it far beyond the China's Southern border, airlines would not consider it.

"If it's a matter of not carrying freight, they tend to accept that quite often on routes," he said. "They can sometimes accept a bit of a passenger limitation, but not [much] because airlines now operate with load factors up in the 90 per cent region and just can't afford to operate less than that."

Four of the nine planes the review looked at on the Guangzhou route were able to leave Wellington with a full load of passengers, but two of those were very new, and one was not yet in service.

That was another new Airbus, an A330-900NEO (new engine option), and the only one able to take off for Los Angeles when the runway was classified as wet, which the airport said happened about 10 percent of the time.

But Irene King, an aviation consultant, said the new aircraft data could be unreliable.

"With some of these aircraft we're looking at performance data that is highly questionable because it's a manufacturer's data only, or the aircraft doesn't exist, so we've got no performance data," she said.

The report's author agreed, noting the new planes may actually be heavier.

Ms King said airlines would not want to spend lots of money upgrading aircraft with more powerful engines, especially on un-proven routes.

"I'd say most [airlines] would just shake their heads and say no, we've got better options to deploy this expensive equipment," she said.

"There would be some pretty severe payload restrictions from time to time. Obviously you can't just randomly work out what your payloads restrictions is going to be on any particular day, so you'd actually have to restrict sales."

Ms King said no low-cost carrier would pick up the route if they could not have a full plane, but said some airlines with high-value business class customers may be able to make the trade-off, or find some other strategic reason to fly into the capital.

Wellington International Airport chief executive Steve Sanderson said the target market was the three major hubs in Asia - Singapore, Guangzhou and Hong Kong - and eight of the nine aircraft can make it with a full payload.

"Four of the nine that were commissioned can also make the full passenger load to the West Coast of America."

He disputed Ms King's point on payload restrictions, and said eight of aircraft could "quite comfortably" take off from the extended runway to the destinations the company had identified.