Footprints of a weasel were found on a tracking card at Rotopiko sanctuary

National Wetland Trust volunteer Noriko Peeters checking a tracking tunnel. Photo / Supplied

Pest detectives have discovered that a weasel has managed to find its way past the pest-fence and into the wetland reserve at Rotopiko near Ohaupo.

The National Wetland Trust, a community operated project, built the pest-fence in 2012 and since then, and up until last week, the only mammal pests that have managed to get through have been the occasional mouse.

Staff and volunteers of the trust regularly check a network of tracking tunnels to monitor numbers of native critters like weta and skinks but also to keep a close eye on any potential fence busters such as the weasel.

Trust executive officer Karen Denyer discovered that there had been an intruder when she pulled an inked footprint tracker card from a tunnel, when she saw what was on it she said her heart sank.

"We'd just had a fantastic day planting and learning about wetlands with Year 3 and 4 students from Ohaupo Primary School. They learned to read tracking cards, and I was telling them we've never had anything but mice get inside the fence.

"Little did I know," said Karen.

Weasel footprint on a tracking card inside the Rotopiko Wetland Sanctuary. Photo / Karen Denyer

Upon finding the footprints the trust immediately checked the fence for holes in which there were none. They also quickly deployed baited traps inside the reserve.

They are still not sure how the weasel got in, they believe it could have snuck in while a vehicle gate was open or through a tiny underground hole that they have yet to discover.

"Weasels are the wee-est of the three mustelid species in New Zealand," said karen.

"If there's a mouse-sized underground tunnel somewhere around our fence it's quite possible a weasel took advantage of it.

"We will keep looking and put out more detection tunnels."