Five times as many West Auckland households - about 4500 - are now in the revised aerial spray zone where a moth pest will be blitzed with a biological insecticide.

The countdown to the Ministry of Agriculture and Forestry's controversial helicopter spray operation for painted apple moth has begun.

The helicopter is expected to start its swoop a week from today.

MAF held a military-style briefing yesterday at its operations headquarters in Pioneer St, Henderson, to outline details of its plan.

The original spray zone was to affect just 800 properties over 300ha but the number is now five times that. Between 4000 and 5000 properties are expected to come within the revised 560ha spray zone.

Along with the major infestation sites at Waikumete Cemetery and Traherne Island in the upper Waitemata Harbour, MAF originally planned to spray only the margins of the Whau River and its tributaries.

But the Whau River area has now been widened to include hundreds more households.

Other targeted areas include Kelston, Glendene and the Avondale Peninsula.

The ministry re-drew the spray zone when it decided the spread of the moth meant a wider area had to be covered. It was unable then to put a figure on how many homes would be affected and was still unsure yesterday of the number of properties the latest plan would affect.

The moth's spread is being tracked by a network of 600 baited traps across Auckland. Catches over the past two months have reached almost 500 a week.

Single moth catches have been made in Scenic Drive in the Waitakere Ranges, in Pt Chevalier and Westmere, Onehunga, Mt Wellington and as far east as Pakuranga.

The biggest player in MAF's efforts to eradicate the moth through the aerial operation will be Auckland's fickle weather.

The operation will probably start at dawn and calm, dry conditions with no possibility of rain are needed for at least five hours after it finishes.

Wind speeds must be less than 12 km/h.

Operations controller Peter Wilkins said the helicopter would stop operating as soon as rain started and it might take three days of flying to complete one spray. "It just depends entirely on the weather."

The biological insecticide Foray 48B, or Btk, will be sprayed at a rate of five litres a hectare, or 2800 litres for one completed spray, at a height of about 45m.

The twin-engined BK117 helicopter will take about seven hours to do each complete spray and it will have to land and take off again for re-loading about eight times.

The helicopter's take-off point is in McLeod Park, Te Atatu South, and its flight plans have been approved by the Civil Aviation Authority, which wanted the BK117 used because of its high safety record.

The Machine uses a Global Positioning System (GPS) to ensure that the same area is not sprayed twice and that spray is not dropped outside the target zone.

Specially calibrated equipment mounted on booms will allow only precisely measured droplets of Btk to be released.

The sprays will be repeated at three-week intervals for between six and eight sprays, but after three complete sprays MAF will assess the effectiveness of the operation.

More than 90 people are on a public register as having special health concerns over the aerial spraying.

MAF's advice to residents is to stay indoors with windows and doors closed rather than leave the area.

"Very few people will need to leave their homes," said MAF's head of the painted apple moth programme, Dr Ruth Frampton.

"Our suggestion is to those who wish to avoid the spray, to stay in their homes for a period of up to two hours following the spray."

Eight radio stations will broadcast updates on whether the spraying will go ahead, at half-hourly intervals from 5.30 am on the scheduled day.

An 0800 number has also been set up with a recorded message either confirming lift-off or cancelling it.

A residents' advisory group, which has been strongly critical of MAF's handling of the painted apple moth invasion, yesterday doubted that the helicopter programme would succeed.

"We want this moth eradicated as fast as possible but we want a properly run operation instead of MAF barrelling ahead with a scheme we believe will fail," said Kubi Witten-Hannah, the group's chairman.

The group will hold a public meeting next week to talk to local residents about the aerial campaign.

Painted apple moth comes from the same family as the white spotted tussock moth, eradicated in Auckland's eastern suburbs during a DC-6 aerial spray operation in 1996-97.

Painted apple moth was first found in Glendene more than two years ago.

It is a native of Australia and a threat to New Zealand's horticultural and forestry industries.

* For further information on MAF's helicopter spraying operation, call 0800 96 96 96.