Volunteers have found evidence of rats inside the Bushy Park Tarapuruhi forest sanctuary for the first time in five years.
A few footprints were found in a corner of the 100ha Whanganui sanctuary during the most recent rodent audit, sanctuary manager Mandy Brooke said.
The endangered tieke and hihi reintroduced to the sanctuary are very vulnerable to rats, and keeping them safe is a number one priority.
Groups of 15 to 20 volunteers have been at the sanctuary twice a week, setting up and monitoring tracking tunnels baited with peanut butter. The tunnels contain cards that record the footprints of animals taking the bait, and there have been a few rat footprints.
Rodent audits are undertaken at least four times a year. The last one was in February, Brooke said.
Traps and poison baits have been laid in the corner of the sanctuary where the footprints were found. Baits are being taken by rats, but none have been caught so far.
Brooke is consulting the Department of Conservation about what else can be done.
There is no sign of a breach in the sanctuary's predator-proof fence where the footprints were found. It was checked regularly during the lockdown.
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The last time rats were found in the sanctuary was in late 2014, and there were more of them across the forest. They were eradicated by February 2015. After that, rodent audits were done every six weeks to make sure they had not returned.
Incursions are common at predator-free sanctuaries and the rats will be re-eradicated, Brooke said.
The only other instance of rats inside the fence was one caught in the forest surrounding the homestead building. That area has a separate internal fence to contain any rat that arrives in a vehicle.
"We trap that space intensively all the time. I think this shows that our monitoring is working and our response is solid."
Having to eradicate rats again is stressful and frustrating, Brooke said.
"I'm mad about it, but it's what we have to deal with right now."