Whanganui resident Allan Wrigglesworth was surprised to find large clumps of what he called "white fat" washed up on northern parts of Castlecliff Beach near Kapiti Terrace on Sunday morning.
Wrigglesworth, who lives nearby and was out for a walk, said he made the discovery around 8am.
"Getting fat up here is unheard of," Wrigglesworth said.
"This isn't next to the [Whanganui] river mouth or on South Beach, this is kilometres away.
"There has been fat on the beach before, periodically. It's usually only 5 millimetre little pellets though.
"Something must have gone wrong somewhere, and it's not from the local fish and chip shop, that's for sure.
"I can only speculate, but it'll be from the tanneries or the freezing works, no one else has lumps of fat that big.
"I've never come across anything this size before, ever.
"Horizons [Regional Council] should be concerned, it's their job to be concerned. If this is happening here, you have to wonder, what else is happening out there?"
A Whanganui District Council spokeswoman said they were informed of the situation early on Sunday morning, and referred the matter to Horizons Regional Council.
A Horizons spokeswoman said an officer responded to "a butter-like substance that has washed up on the beach in Whanganui".
"We are not sure what it is or where it has come from."
Samples of the substance had been taken for analysis, the spokeswoman said.