The Department of Conservation (DOC) is unsure how an area of the Takitimu Conservation Area will recover after a farmer allowed cattle to graze there illegally.

DOC said about 380 cows were in part of the conservation area for about two weeks last year.

In a statement on Monday, DOC said Wairaki Station Ltd was sentenced in the Invercargill District Court on February 20 after admitting one charge of "knowing and without authority carrying out an activity, namely farming operations, on a conservation area".

STUFF DOC said about 380 cows were illegally let in to part of the conservation area for about two weeks last year.

About 380 heads of in-calf cows were grazing in two areas that covered about 190 hectares for 15 days last year.

The area had been retired from grazing in the early 1990s.

In an affidavit, Wairaki director Richard Slee said he saw the area as "rough feed".

Judge Callaghan imposed fines of $52,450.

DOC acting operations manager John McCarroll said the Takitimu Conservation Area had significant conservation values and the cattle caused considerable damage and impacts including stock tracking, heavy pugging of stream riparian margins and wet areas, and vegetation effects.

"How well this area recovers is unknown at this stage. We do know that this type of stock incursion adds to the persistence of weeds, gorse and broom, which DOC has been actively controlling.

"We have been working with Wairaki Station in relation to managing these impacts and will continue to do so over the next three years."