A suspected ammonia spill that forced Fonterra's Maungaturoto dairy factory to be evacuated was a false alarm.

The dairy giant confirmed to The Northern Advocate this afternoon that there was no risk to staff or the local community following the evacuation, in which all staff were removed from the site after the alarm was raised at 1.30pm.

Emergency services rushed to the small town on State Highway 12, in the Kaipara District following an alarm activation, which was believed to be an ammonia leak.

The site was on high alert at 2pm, with concerns that the town itself, a few kilometres west of the factory, might also have to be evacuated.

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A Fire Service spokesman said at the time that the wind coming from the northwest was blowing the suspected ammonia fumes away from the town, so evacuating the town might be avoided.

However, it turned out to be a false alarm, a Fire Service spokesman said later.

All emergency personnel have now been stood down and workers have returned to the factory.

A spokesman for Fonterra said there was no ammonia leak, and the alarm was set off when a power generator was switched off.

"The alarm was activated at our Maungaturoto site due to generators being switched off for annual full-site maintenance," he said.

"Given the site is not on product, the ammonia we use as a coolant is locked away for the season - there is no risk to staff on site or to the community."