Damage from a fire at the Meadow Mushrooms site at Prebbleton south of Christchurch will put some of the operation out of action for several weeks.
The Fire Service says the blaze broke out in an outdoor storage at the firm's Springs Road plant at 6.20pm on Thursday.
Area commander John Graham says the blaze spread to buildings, scrub and a nearby hedgerow and fresh fires from smouldering embers sprang up all night, fanned by norwesterlies.
About 40 fire fighters with at least eight fire engines and other specialist vehicles fought the fire in and around the building. Two fire fighters received minor burns.
Mr Graham says conditions in Canterbury are tinder dry and high windscan easily fan small fires out of control.
Meadow Mushrooms Ltd board chairman Philip Burdon says about a third of one major complex has been affected, and it will need to be rebuilt.
He says there has been significant damage to the tray farm and the spawn growing areas.
Mr Burdon says they will be able to improvise to get that operation back up and running within a couple of weeks.
He says doesn't expect staff from that site to be too affected, however there may be reduced picking hours for several weeks.
Fire safety inspectors will try to determine the cause of the fire.