Ballance Agri-Nutrients has been fined $82,500 for the unauthorised discharge of contaminants, namely sulphur dioxide, from its fertiliser manufacturing plant in Mount Maunganui last year.

Ballance pleaded guilty to the charge, which related to an incident on May 22, 2017, where a build-up of fumes was discharged into the air.

Workers at the Ballance site described seeing a "huge gas cloud" heading towards the Mount, 20 to 30m wide, while others described it as "a plume of black smoke coming towards us".

The gas cloud drifted from the roof of the manufacturing plant and across the Hewletts Rd site towards Totara St.

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Several people at the site and working nearby were exposed to the gas cloud.

They experienced coughing, a "funny taste" in their mouths, irritation of the eyes and respiratory system, but their symptoms were relatively short-lived.

Two Winstone Transport truck drivers were treated and discharged from Tauranga Hospital after struggling to breathe.

In the Environment Court in May this year, the company's chief executive Mark Wynne pleaded guilty to a charge of discharging a contaminant, sulphur dioxide and/or fluoride, into the air.

The summary of facts showed there was a breakdown in communication between controllers during a shift change, in regards to the conveyor speed processing product.

This resulted in a build-up of fumes and about 7.30am the cloud of fluoride and sulphur dioxide fumes bypassed the treatment systems and was released.

The new shift operator was unaware the fumes had been discharged. People outside the plant tried to contact the controller on VHF radio but he did not respond, and the plant was shut down about 7.50am.

Victoria Brewer, the Bay of Plenty Regional Council's lawyer, told Judge David Kirkpatrick in May that the defendant had two prior convictions in 1999 and 2015 for similar offending.

Ballance Agri-Nutrients lawyer Janette Campbell said Ballance's two earlier convictions were unrelated to last year's discharge.

In his ruling, Judge Kirkpatrick said a key issue presented in the case was the degree to which Ballance's previous convictions bear on the level of fine that ought to be imposed.

He said it was important to keep in mind the previous convictions related to a different plant at the site.

"However, the distinction may not be valid where the systematic nature of the current offending is the same or similar to that in the previous case."

Kirkpatrick found that, from the material presented to him, the problem was similar to the previous cases at the site where discharges occurred after shut-down periods for maintenance when the plant was re-started, and operating staff did not adequately check the plant, resulting in the discharges.

"I, therefore, accept the prosecutor's submission that the offending here is similar in seriousness to the two previous offences, being moderately serious, resulting in temporary health effects to people and near the site and revealing shortcomings in the defendant's operations and emergency processes."

Kirkpatrick also noted that the regional council, as consent authority, was undertaking a review of Ballance's resource consent.

"That appears to me to be a highly desirable action".

Ballance chief executive Mark Wynne told the Bay of Plenty Times the company had invested to make sure the incident was not repeated.

"We accept the fine. It was potentially a very serious discharge and acknowledge right up front that it should not have occurred. And we completely apologise to anyone that's impacted from it."

Wynne said Ballance accepted that communications across the shift handovers were less than ideal and had since improved that.

Ballance had also put in improved additional safeguards, including an automated shutdown.

"So that this situation can never be repeated again," Wynne said.

He said the previous incidents at the same site occurred for very different reasons, "so they're not directly connected".

Last year Ballance invested over $8m on a new sulphur dioxide converter.

Read more: Millions spent at fertiliser works to improve Tauranga air quality

No one working on March 22, 2017, had been moved on or had lost their job as a result of the discharge, Wynne confirmed when asked by the Bay of Plenty Times.

He said specific training had been implemented and a full end-to-end review of procedures had been done, particularly focused around communications at shift handover time.

Regional Council compliance manager Alex Miller said things like sulphur dioxide and fluoride could have significant effects on both the environment and human health, "so it's critical that commercial operations that generate these contaminants are well managed".

"While this event was an accident, caused by a series of fundamental errors at the fertiliser plant, with a main road on one side and the Whareroa community behind on the other, the fact that more people were not affected was a matter of good fortune rather than good management," he said.

"We hope that the measures which they've put in place after the offending mean that we don't arrive here ever again."