The mystery of how a weasel got inside Wellington wildlife sanctuary Zealandia could be solved.

Last year the pest was Zealandia's first incursion in 10 years, sparking a mission to find and destroy it. But how it got inside remained unknown.

It seems it could have entered the sanctuary through a passageway of rotting tree roots, Zealandia conservation manager Dr Danielle Shanahan said.

"When the fence was being built, large pine trees were cut on either side of the fence to ensure there would be no vegetation to create a bridge for predator species to get into the sanctuary.

SUPPLIED The female weasel when captured and killed.

READ MORE:

* Zealandia could set 170 traps for weasel

* Zealandia weasel caught after over a week of trapping

* Predator inside Zealandia remains at large

* Zealandia weasel caught

"Stumps with root systems were left behind, and while we do not know for sure if there has been a tunnel created by these decaying tree roots, this was a potential mode of entry for the weasel."

Despite the new theory, Shanahan said they might never be 100 per cent certain how the weasel got inside.

Since the discovery, Zealandia had an ongoing work programme to ensure that any potential tunnels were filled in, she said.

"So far, we have not found any that go under the fence, but we are assessing all stumps near the perimeter to make sure we don't have a problem."

Last year the pest was Zealandia's first incursion in 10 years, igniting a mission to find and destroy it (file photo).

The exact entry point remained unknown, she said.

"We don't know exactly where it got into the valley, but the footprints were discovered and the weasel was later caught in the southern tip of the sanctuary."

Original speculations guessed the weasel could have been dropped by a predatory bird flying past, like a harrier.

The 8.5km-long fence was meticulously checked for tiny holes.

Zealandia set around 110 traps baited with rabbit meat and eggs, with a further 60 traps at the ready, and the weasel was caught just over a week later.

ROB SUISTED Zealandia's 8.5km-long fence was meticulously checked for tiny holes.

Camera traps also captured the weasel on video three times at the same location.

In the week that the weasel roamed the sanctuary, it seemed it didn't harm any native birds.

"Happily we did not see any evidence of damage from the incursion," Shanahan said.

"We do not think we will ever know for 100 per cent how this animal got in, but we covered all bases and checked our systems to make sure they were robust.

"This is why our routine pest audit systems are crucial to ensure that we are always checking and ensuring we do not have a repeat incursion."