Fat and wet wipes are what led to two wastewater overflow incidents in Rotorua last weekend

Warnings were placed by three Rotorua streams following wastewater overflow incidents last weekend. Photo / Andrew Warner

A mix of fat and wet wipes are what caused two wastewater overflows into Rotorua streams last weekend.

Rotorua Lakes Council advised the public to keep clear of three streams on the weekend after two separate incidents.

It warned people on social media wastewater may have entered Otamatea Stream via the stormwater network on Petrie St and possibly entered the Utuhina Stream below Martin St.

The second incident was at Linton Park, near Deborah Pl, and it was likely some wastewater entered the Mangakakahi Stream.

Since then the council worked to figure out the cause, and updated its website today.

"During the weekend two blockages within the wastewater network caused a back-up of sewage that overflowed from manholes, onto land and into nearby stormwater inlets," the statement on the website said.

These blockages were made up of a mixture of solidified fat and cleaning wipes "often referred to as 'fatbergs'."

Infrastructure Networks Performance Manager Eric Cawte said damage to systems and faults were expected from time to time.

"However, in these cases, the cause is something that is largely avoidable."

He said it was a timely reminder to the community that placing foreign items such as wipes into the wastewater network, even those that say 'flushable' on the packet, can result in significant issues for the infrastructure and, more importantly, for the environment.

He said council and Trility staff responded to the overflow events as soon as they were alerted on Saturday, and worked to clean and disinfect the affected areas and installed signage to warn the public about the potential health risks.

Daily sampling was undertaken and results showed water quality was back within the recommended safe bathing levels for the past two days.

Testing at the Mangakakahi Stream earlier in the week showed preliminary results of elevated bacteria levels.

The district council was liaising with Bay of Plenty Regional Council and Toi Te Ora Public Health about the removal of the warning signage.

Cawte said when wastewater overflows out of the network, the natural course of flow would take it to the lowest ground levels, such as streams, which were part of the natural stormwater system.

"Unfortunately, wastewater flows the same way as stormwater. That means when wastewater overflows from the system it is naturally likely to end up in a waterway.

"We need to work together as a community to ensure that we do all that we can to avoid blockages, and keeping foreign substances and objects out of the system is the number one way to do that."