A threatened native bird might be thriving on a Marlborough mountain had it only been saved from rats with 1080 poison, a veteran conservationist says.

Peter Gaze oversaw a team of Department of Conservation (DoC) rangers working to boost mohua (yellowhead) numbers on Mt Stokes in the late 1980s and 1990s.

When local rat numbers exploded at the turn of the new century, the mountain's fragile population of the tiny, canary-coloured native species was lost forever.

Gaze was highlighting the case now to warn what could happen in a "mega mast" – a major forest seeding event that littered the ground with food for a plague of pests – if vulnerable populations weren't shielded.

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DoC was now preparing to respond to the biggest such mast in nearly half a century, with an unprecedented, $38m pest control operation spanning a million hectares of conservation land.

Priority sites included Kahurangi, Abel Tasman, Arthur's Pass, Westland, Mt Aspiring and Fiordland national parks, the Catlins and Whirinaki.

More than 66,000ha would be covered with trapping, and the rest with aerial 1080 poison drops.

Yet many areas would have to be left unprotected – and DoC scientists expected some localised extinctions as a result.

Among those most vulnerable species were orange-fronted parakeet, long and short tailed bats, rock wren – and mohua.

Gaze explained that five mohua were discovered high on Mt Stokes in 1985.

DoC controlled stoats with a network of traps on the mountain, and adult birds increased in numbers and had breeding success.

By 1999, they numbered over 90 birds.

But the best-practice trapping programme was overrun by rat plagues during the 1999-2000 beech mast, and by the end of 2000 the birds that rangers had worked so hard to protect, were gone.

Gaze said he was adamant the mohua could have been saved if aerial 1080 had been applied.

"That's the big lesson that came out of it," he said.

"Trapping is great when you're only dealing with stoats. But when you're dealing with a bird that's also susceptible to rats, you just can't cut it with traps."

"Knowing what we know now, and having the tools we've got now, we could have applied 1080 right around Mt Stokes in the early stages of that mast."

"We could have knocked the rat population down. The mohua would have had a cracker breeding season. We'd still have mohua on Mt Stokes."

Forest & Bird's chief conservation advisor, Kevin Hackwell, compared the plight of the mohua on Mt Stokes with the thriving population in the Landsborough Valley in south Westland.

There, DoC had carried out an intensive predator control programme over 21 years, including six 1080 operations.

Local Mohua today numbered 444, up from 14 when monitoring first began, and were now the most common bird counted in the valley.

"This is what aerial 1080 can do for our most vulnerable native species," Hackwell said.

"Trapping alone can't achieve these results."

Hackwell said it was good news DoC was launching a large-scale operation, but the department was nonetheless constrained.

"There are still many areas of important habitat that will miss out. We need even more funding and resources for this and future mast events, so we don't see a repeat of what happened on Mt Stokes."

DoC's national operations director, Hilary Aikman, said staff chose which populations received the most protection through an assessment that looked at whether the species was likely to go extinct, what pest pressure they'd face, and whether the predator control would even be feasible.

"The size of the programme is limited by resources and the capacity of DOC and our contractors to deliver the work within the required timeframes for it to be successful."

In this year's case, she disagreed preparations could have been better organised.

"DoC has been planning for this year's programme for over a year, following the prediction of this year's beech mast from climate models at the end of the 2018 summer," Aikman said.

"These predictions were confirmed by our seed sampling work in February and March this year."

The secure funding for predator control – an extra $81m over four years was injected into last year's Budget – had enabled DoC to plan "well ahead" and respond to the mega mast.

"In addition, the relatively small predator control programme in 2018 freed up staff to focus on planning this year's large programme."

DoC was planning to expand the area under sustained predator control - where predators were kept to low levels under a regular cycle of control work – from 800,000ha to 1.85 million ha, or to 20 per cent of conservation land, by 2022.

"This will reduce the risk of losing local populations of species."