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State-of-the-art "quiet" aircraft may be the answer to noise pollution in one of New Zealand's leading tourist destinations.

Some tourism operators who fly sightseers over the scenery of Fiordland are investing in aircraft that operate with much less noise in response to authorities cracking down on the overhead sound that can ruin the experience for visitors on the ground.

However, other operators warn the high cost of quieter aircraft is too risky when rights to fly in Fiordland are up in the air.

There are up to 200 aircraft landings and take-offs from the Milford Sound airstrip on the busiest days. The rules for flying in the area are still to be determined by the Fiordland National Park Management Plan, due out in about six months.

Queenstown's Over The Top helicopter company has just bought a $3.5 million "environmentally friendly" Eurocopter B4, the first to be used in a tourist operation in New Zealand.

A fully enclosed tail rotor system means it runs substantially quieter than usual helicopters.

"The new machine has ... been adopted worldwide by the aviation fraternity as the quietest helicopter available on the market," said Over The Top director Louisa "Choppy" Patterson. "That's of great importance to our company as we fly over and into some of the most environmentally sensitive areas in New Zealand."

Last year, Queenstown-based Air Milford last year bought a quiet-engined Cessna Caravan fixed-wing plane to try to guarantee its right to fly over Fiordland in future.

The Department of Conservation (DoC) has been advocating the use of quieter aircraft over national parks. Community relations manager Martin Rodd applauded the addition of the Eurocopter. "Aircraft provide an outstanding opportunity for visitors to gain an appreciation of the spectacular natural landscapes and remoteness of Fiordland National Park," he said.

"Managing the effects of aircraft noise on other park users, like those tramping the Milford Track or exploring its wilderness areas, is the challenge."

The Queenstown Milford Users Group, which represents aircraft operators, said all operators wanted to adopt quieter aircraft, but the cost was a big issue.

Spokesman Lloyd Matheson said restrictions to be set on flying and landing in Fiordland in the management plan meant there was no guarantee the "mega-bucks" spent on quieter aircraft could be recouped.