Emergency services were called to the sit on Horotiu Rd just after 9.30am.
Northern fire communications shift manager Craig Dally said seven vehicles attended the incident.
He said there was no fire or injury but there was a loud explosion and smell of gas.
Dally said fire services are investigating the cause of explosion.
Open Country Dairy chairman Laurie Margrain said there were no injuries and it appears the incident was caused by a fault in an air heater.
Margrain said they are assessing damage and redirecting milk to the Waharoa factory for processing.
Meat Processor AFFCO, which operates a large plant next door to the Open Country factory told RNZ there had been no disruption to there operations as a result of the explosion.
