Fire crews have now left the scene of a large fire in Christchurch.

A shed at a Fulton Hogan branch in Halswell Junction Road in Islington caught fire early this morning.

The Fire Service was alerted to the blaze at 3.35am after a courier spotted flames coming from the structure.

Shift manager Brent Dunn says the shed has been extensively damaged.

He says when crews arrived, it was well ablaze.

Brent Dunn said it was 40 by 15 square metres and half of it was on fire.

It was unclear what caused the blaze but he said it was not being treated as suspicious.