UPDATED 4.15PM: Eighteen fire engines and a helicopter with a monsoon bucket are battling a large scrub fire in south Canterbury.

A shed has been destroyed by the one kilometre-wide fire on the corner of Old West Coast Road and Chattertons Road near West Melton, but there've been no reports of injury.

Fire Service shift manager Brent Dunn says the fire started in a rubbish pit and got out of control.

He says fire crews have the blaze surrounded and are working to contain it.