Auckland's Watercare says about 24 companies each month are caught breaching their trade waste consents, but it won't reveal their names.
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Auckland Council controlled water network operator initially refused to answer any questions about trade waste consents, citing "privacy issues", as part of RNZ's investigation into companies caught dumping contaminants down the drain.
However, following pressure from some councillors to release the information Watercare today admitted a "monthly non-compliance rate of around 3 percent" among the 800 trade waste consents it monitors.
"The breaches are usually minor and are quickly resolved. This is because we undertake regular monitoring and detect small issues before they have the chance to escalate," it said in a statement.
"When there is a breach, we require the customer to investigate what happened and explain why their wastewater was not compliant. They outline what they have done to resolve the situation and their actions to ensure it won't happen again. We also retest their wastewater to ensure compliance."
It had developed "strong working relationships" with these companies and did not believe it should name those who had been non-compliant," Watercare said.
"In recent years, our customers have not had any significant non-compliance incidents. This means our customers have not caused us to breach resource consent conditions at our wastewater treatment plants. If a significant incident did occur, then we would prosecute and name the offending company."
"Given the success of our current way of managing trade waste compliance in Auckland - an approach based on mutual trust and working together to achieve good outcomes for our wastewater assets and environment - we see no benefit in naming customers that experience minor non-compliance and undertake work to remedy the situation," it said.
RNZ has asked the Ombudsman to investigate, considering some councils fully disclosed information about consent breaches, including in some cases, even giving every non-compliant test result.
Watercare has not prosecuted any company for trade waste breaches in the last 10 years.