Mystery Whanganui beach substance: Will lab test reveal all?

The substance washed up at Castlecliff Beach on August 8. Photo / Lewis Gardner

The source of a buttery, fat-like substance that washed up on Whanganui's Castlecliff Beach remains a mystery, but laboratory tests will soon reveal its identity.

Samples taken were being tested by an independent laboratory, Horizons Regional Council consents monitoring team leader Pita Kinaston said.

Horizons received a complaint about the wash-up at 9.45am on August 8 via its Pollution Hotline, with a pollution response duty officer arriving at the Morgan St beach access about an hour later.

The samples were collected on the northern beach at the North Mole.

Kinaston said the officer found the substance at Castlecliff Beach at the Rangiora St and Seafront Rd end, while a small amount was also at the Longbeach Drive access.

"There was no fat-like substance at South Beach or Kai Iwi Beach," he said.

"The officer was joined by another officer and they inspected stormwater outlets from the industrial area along the true right of the Whanganui River mouth.

"The fat-like substance was not observed anywhere along the stormwater outlets or Whanganui River mouth."

Horizons had inquired with Whanganui District Council and several industries into the source of the substance, Kinaston said.

"All parties advised the fat-like substance could not have come from their premises.

"Horizons has also contacted the Taranaki Regional Council, who advised they have not been notified of any spills further up the coast."

The samples were being tested by an independent laboratory to confirm what the substance was, Kinaston said.

He said a Horizons officer was back on the beach on Tuesday to "assess any ongoing effects".