Airlines are worried about the potential financial fallout of the planned $300 million extension of Wellington Airport's runway.

Pilots are also alarmed at the lack of safety zones at the end of the runway and have filed court action which they say if successful could affect the economic viability of the project.

Cost benefit analysis done for airport shows expansion would allow long haul flights will connect the capital to 65 per cent of the world's population and provide a $7 return for every $1 invested.

However, the Board of Airline Representatives' chief executive John Beckett says there are risks.

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"If they go ahead and they make a huge loss it will be recovered by increasing the charges on all other services into Wellington."

Air New Zealand says it has been clear that its business strategy does not include operating long haul flights from Wellington Airport.

"We also want to be reassured that that customers who do not use the extended runway will not be asked to help pay for it," an airline spokeswoman said.

Beckett said his organisation had just received the airport's analysis which would now be reviewed by economists from the NZIER.

Barnz would meet airport representatives on Monday to discuss the plans.

Becket said although Barnz would not necessarily be informed of any member airlines contemplating flying to Wellington, he knew of none considering it.

A Wellington airport spokesman said route decisions were made in a highly competitive environment and it had never known of Barnz being made aware of new route decisions in advance of public announcements.

Airport chief executive Steve Sanderson said extending the runway about 330m would open up direct links for long haul aircraft to Asia and North America within 12.5 hours flying time.

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"When Wellington extended its runway in 1972 to enable jet services to Australia there was much public debate about whether the benefits and costs stacked up, but we now have up to 70 return international flights per week and the benefits are well established. Imagine where Wellington might be today if that hadn't happened."

Forecasts show there would be one long haul service a day in 2021 growing to 4 flights per day by 2035.

The international visitor growth for New Zealand is forecast be an additional 125,000 visitors per year by 2035 growing to 200,000 a year by 2060, the airport said.