A decade-long pest-busting campaign has begun bringing back rare grey-faced petrels to a Waikato mountain once smothered in seabirds.

In ancient times, thousands of seabirds each year flocked to the steep cliffs of Karioi, just south of Raglan.

But an onslaught from pest predators over past centuries have stripped them from their traditional nesting sites on the side of the mountain.

While the birds still return - Karioi is one of the few remaining mainland sites where they're known to breed -until recently, no chicks had been known to survive into adulthood.

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That could be about to change, thanks to years of intensive pest control operations that have attacked pests from ground and air.

Possum control by the Department of Conservation (DoC), Waikato Regional Council (WRC) and the local Tainui hapu has boosted the efforts of the volunteer project which started in 2009, when volunteers put out traps in Upper Wainui Reserve.

At the start of the campaign, the trapping was so intense that volunteers were running checks for killed pests several times a week.

Today, pest numbers have dropped greatly, and 45 volunteers are still managing 900 traps across 2300ha.

The hapu team cut new trap lines across the entire mountain to install bait stations across 240ha of difficult and dangerous terrain.

They've also worked with council contractors to carry out pest control work on a further 2000ha of surrounding private land.

DoC, meanwhile, was beating back possums with aerial 1080 drops and work on the ground.

In a promising sign, the Karioi Maunga ki te Moana biodiversity team last year discovered seven healthy chicks in the deep burrows.

A grey-faced petrel chick. Few have survived to live to adulthood at Waikato's Mt Kairoi. Photo / Graeme Taylor
A grey-faced petrel chick. Few have survived to live to adulthood at Waikato's Mt Kairoi. Photo / Graeme Taylor

"Aerial 1080 has been the most effective method, with a recent operation including 1600ha of public conservation land," DoC biodiversity ranger Cara Hansen said.

Operations that WRC was managing on about 2600ha of surrounding private land had driven possum numbers down to "almost undetectable levels".

Staff from West Coast Health are among groups that have helped with the trapping operation. Photo / Supplied
Staff from West Coast Health are among groups that have helped with the trapping operation. Photo / Supplied

"A knockdown of rat and stoat numbers will allow many of our native forest birds to have a successful breeding season this year," Hansen said.

The work is set to be carried on with a new sustainability and ecological restoration programme being offered to local students.