A Hawke's Bay stream was left "running green" after 80,000 litres of dairy effluent flowed off a farm and into a popular swimming spot.

The Environment Court in Hastings heard on Tuesday it could all have been avoided if the farm's acting manager had been trained in how to operate an irrigator.

Patoka-based dairying company Maxwell Farms Ltd appeared after pleading guilty to a charge of discharging effluent to land where it could enter a waterway.

An estimated 144,000 litres of effluent was discharged from an irrigator, of which 80,000 litres entered the Mangatutu Stream - a tributary of the Tutaekuri River - on December 23, 2019.

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Downstream water samples showed ammonia was present at 90 times the allowable levels.

Hawke's Bay Regional Council launched an investigation, which led to an issuing of public warning not to swim in the popular swimming spot for a week.

The court that an irrigator set was not checked and had been left running for hours longer than it should have.

The council became aware only when a member of the public informed the pollution response team that a stream was "running green", council lawyer Nicola Graham said.

Graham claimed the incident was a direct result of a failure of management to ensure the correct processes and training were in place.

The acting farm manager had told council he'd not been formally trained how to use the irrigator and had only watched other employees use it, according to Graham.

She quoted environmental, societal and cultural impacts from the incident, and said the company's actions were "reckless in the least".

She noted that employees of the company had been issued infringement notices twice since 2015 for dairy effluent offending.

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The effluent was discharged into the Mangatutu Stream - a tributary of the Tutaekuri River - on December 23, 2019. Photo / File
The effluent was discharged into the Mangatutu Stream - a tributary of the Tutaekuri River - on December 23, 2019. Photo / File

Maxwell Farms Ltd's lawyer Simon Connolly said while "regrettable", the offending was primarily down to "human error".

Connolly said management was unaware the staff member, who is no longer farm manager, had not received formal training.

Graham sought a starting point for a fine of $100,000, while Connolly said it should be $40,000.

Judge Melinda Dickey reserved her decision. It will be delivered next month.