The Brook Waimarama Sanctuary chief executive Ru Collin says pressure on the predator proof fence is "business as usual".

Nelson's Brook Sanctuary has missed out on its first kiwi population because a surge in rat numbers saw some breach the fence.

Brook Waimarama Sanctuary chief executive Ru Collin said an untimely incursion of rats in September, fuelled by this year's mega mast, put paid to a planned introduction of rowi kiwi.

Collin said it was a "Hobson's choice" as if they hadn't set traps to get rid of the rodents the reintroduction could have gone ahead.

"You can't have a sanctuary full of rats or you just don't have a sanctuary. It was a tough decision," he said.

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Rats were first detected inside the sanctuary on September 22 and Collin said they had been on "high alert" ever since, with over 3000 devices monitoring or trapping rats.

Supplied The world's 300th rowi chick was successfully hatched at the West Coast Wildlife Centre in Franz Josef this year, but it will be years before the Brook sanctuary has the chance to host one. (File photo)

"Bearing in mind that we usually have sanctuary surveys that last for six weeks and we might do them two or three times a year ... we've left our survey open since September, and we'll leave it open through to past Christmas because of this exceptionally high pressure period."

He said there had also been an incursion of weasels, but volunteers had killed them and there had been no evidence of a lingering weasel population. He said no rats had been detected for the past seven weeks, but they were still proceeding with extreme caution.

"The seed drop which has been described as a mega mast ... one of the larger ones in the past 40 years, that results in very high pressure of predators outside the fence trying to get in."

Collin said incursions were "business as usual" at the sanctuary.

LUZ ZUNIGA The "thin, small line" of defence against predators has taken a battering during this year's mega-mast. (File photo)

"We're constantly, constantly dealing with incursions, it's within the natural environment in that terrain: we've got branches falling through things, rock falls, we're quite sensitive about the sheer volume of people moving in and out of the fence, vehicles, and with such high pressure on the outside it's like a Maginot line, it's just this one thin, small line to stop the very hungry populations outside getting inside."

He said there was a team of about 75 volunteers conducting an extended survey monitoring traps and devices within the sanctuary, and another team checking all 14.5 kilometres of fence for holes as small as 5mm, which mice could squeeze through.

Any introduction of rowi was now likely to be at least a few years away, thanks to the cyclical nature of the breeding programmes. Collin said instead, the sanctuary was shifting focus to the next species on their list for reintroduction: tīeke (saddleback) and kākāriki (orange-fronted parakeet).

"With luck we are hopeful that we might be able to announce release dates before the end of our financial year, so sometime perhaps in the first quart of next year we might be able to announce some good news."

SUPPLIED/KINT With the introduction of kiw off the table for now, the sanctuary is changing its focus and hopes to set a date to reintroduce tīeki/saddleback next year. (File photo)

There were several other species that the sanctuary was hoping to reintroduce, including kākā and mohua (yellowhead), but he said the preparation work was just beginning "to make sure we're 12 to 18 months ahead of ourselves".

Collin also told the Nelson City Council's environment committee last week that the footbridge over the old dam in the sanctuary was nearing completion, and should be completely ready for public access early next year.

The bridge allows for wheelchair users or people with other disabilities to complete a loop track through the lower levels of the sanctuary.