Wellington wildlife sanctuary Zealandia is ramping up its efforts to find a weasel that's invaded the fully-fenced area.
"This is a big deal for us - the last time this happened was over 10 years ago now and mustelids can do so much damage to New Zealand wildlife," Zealandia conservation manager Danielle Shanahan said.
Footprints from what's believed to be a weasel were found 100 metres west of the southern tip of the valley during an annual pest audit on Monday.
On Thursday, a weasel was sighted closer to the entrance and as a result, staff have broadened the focus of their search, Ms Shanahan said.
Ms Shanahan said hunting down the predator among 225 hectares of bush was a difficult challenge but one she was confident staff would achieve.
"New Zealand conservationists have been working on these problems for a very long time so we're getting the best advice possible and we have a lot of experience behind us in New Zealand in eradications."
The entire perimeter of the predator-proof fence has been checked for signs of damage by staff and volunteers.
"We're going to be repeating that check but at this point I'm assuming that the fence is OK," Ms Shanahan said.
"It might have been carried by a harrier or some other bird of prey and dropped into the sanctuary, it could have come in through a gate; vehicle gate for example."
A spokesman for Zealandia said a deliberate introduction was an unlikely but possible cause.
Security cameras have been placed around the sanctuary and 110 traps have been deployed and baited, with hopes the pest will get used to them before they're set by staff next week.
Ms Shanahan said wildlife is being monitored closely with breeding season underway.
"If there is damage, I'm hoping to detect it quite quickly," she said.
Ms Shanahan said the silver lining has been offers of help that came through immediately after news of the incursion broke.
"The Department of Conservation, Greater Wellington (regional council), Wellington City Council... so many more people - it's a massive thing for Wellington to be so proud of its natural conservation and heritage."
Predator Free New Zealand Trust's Jessi Morgan said the incident highlights the challenge the Government has ahead with its aim for the country to be free of rats, stoats and possums by 2050.
"Until we've got a tool to really eradicate predators completely it will mean that we'll have to maintain constant vigilance on the traps and toxins to ensure rat and mustelid numbers don't come back," she said.
Ms Morgan said it will be difficult to protect areas like the Southern Alps from re-invasion, if eradication is achieved.
She said Zealandia's incursion was a good reminder for neighbouring streets to play their part in controlling pests.
"Then it means the fence is the last line of defence as opposed to the first line of defence," she said.
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