Fonterra is limiting the number of staff at its Te Rapa dairy processing site as it works to resolve the cause of a major ammonia leak.

The big 51-year-old site has been in winter shutdown since June 2 and isn't processing milk, but employees due to return to work by now after seasonal maintenance have been told not to come back yet.

The company has confirmed it is continuing to limit staff on site to those required while checks are completed and the cause of the dangerous leak on June 26 is fully resolved.

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The overnight leak, which sparked an emergency response by up to 20 firefighters and forced the site to be evacuated, occurred during scheduled maintenance and created an issue with water quality, the company told the Herald.

It was raining heavily when the leak - initially in liquid form which turned into a highly dangerous gas, according to emergency services - occurred, meaning rainwater which washed into the site's water storage ponds had high pH levels due to it meeting ammonia, the company said.

"We're putting that water through our wastewater treatment systems to neutralise it.

"Given the site is currently shut and undergoing maintenance anyway, we are not rushing this work. The safety of our communities, employees and the environment is our priority."

The situation did not impact the pay of any employee, the company said.

WorkSafe, the Waikato Regional Council and Fire & Emergency NZ have been involved with the situation.

The company did not respond to Herald questions about the location of the leak or the costs of the incident and its aftermath.

The new dairy season started on June 1 but milk production will be limited until calving is over next month.

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Fonterra said the Te Rapa site, which employs 500 people full-time at the season's peak, is not scheduled to fully reopen for several weeks - depending on milk supply.

The Te Rapa site at Horotiu, north of Hamilton, is one of eight Fonterra processing sites in the Waikato.

At a recent hearing where it opposed a major housing and water adventure park development proposal in the area, Fonterra said the site was a critical asset with a replacement value of nearly $1 billion.

In the dairy season it operates 24/7 and processes more than 7.5 million litres of milk a day into milk powders and cream products.

A $20 million expansion of its cream processing plant was completed two years ago.