Wellington police have a slippery problem to get to grips with.

Thieves have made off with a 100kg worm farm from Zealandia's eco-sanctuary in Karori, close to the city.

Senior Constable Dave Ross said the farm contained more than 8000 worms, was full of compost, and would have taken two people to carry it away.

"I've never come across anyone who's taken a worm farm," he told the Dominion Post.

Police believe the thieves may have been after fertiliser for an illegal crop-growing operation.