The historic Pauatahanui General Store, built in 1861, has been badly damaged by fire.
Firefighters were called to the blaze in Paekakariki Hill Rd about 11.30am on Sunday.
Porirua senior station officer Barry Murphy said the fire started in a deep frying vat in the back of the building.
It took firefighters about 40 minutes to bring the blaze under control and a further two hours before the fire was totally out.
"There's a bit of fire damage and the kitchen area is pretty well destroyed," Mr Murphy said.
The dairy owners were visibly upset and declined to comment.
Diane Boyack, owner of nearby Rural Trading Post, said she spotted the smoke and called the fire brigade.
She said the owners were in shock, but the community would help them through.
- Kapi-Mana News