Free Christmas trees are on offer in Christchurch this festive season – and it's helping the environment.

A Lyttelton community group is helping families cut down wilding pines littering the Port Hills to take home and decorate.

And there's another benefit to spreading the Christmas cheer – clearing the pest pines to make way for native trees.

Once the trees are cut, poison is placed on the trunks to stop them from regrowing.

"What we've noticed is that the pine trees, the juveniles, they grow in clusters and so there must be some light getting to that area," the Lyttelton Reserves Committee's Kirsty Brennan told 1News.

"What we'll do in the next planting season is we'll put some natives in there and start expanding the native planting in the reserve."