Is it a bird, is it a plane, or is it something else entirely?

Just what is the source of the mystery oily substance that's splattering the windows and cars of residents in a quiet Taranaki suburb?

Residents of Mouatt St, Waitara, say the problem has been ongoing for years but has got worse in the past eight months.

LIAM COURTENAY Something is leaving a yellowish spotted coating on property which is hard to shift.

The seaward side of their houses and any vehicle left outside ends up splattered by droplets of a yellow-orange substance that is difficult to wash off using water alone.

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Rex Brunning said the problem had increased recently.

LIAM COURTENAY Rex Brunning has to clean his cars every day to remove the mystery spots.

"It appears to be oil-based and it leaves an oily residue if just water is used to try and remove it," he said.

"It's not just a one-off thing, it's almost every day.

"You put the windscreen wipers on and they just go straight over the top of it."

Rex and his wife Kay said they'd been told the substance could be aviation fuel being dumped by planes as they came into land at New Plymouth airport.

"They reckon they just dump it and it's just like a spray coming in from over the sea," Kay said.

But Air New Zealand said this could not be the case.

"The turboprop aircraft that operate to New Plymouth airport do not have the ability to jettison fuel during flight." a spokeswoman said in an emailed statement.

The couple are considering getting the splatters tested to try and establish what it was.

"After all of these years it would be good to find out what the hell it is," Rex said.

Neighbour Craig Midgley said he had experienced similar problems with the substance splattered down the seaward side of his house and vehicles.

"It gets on your vehicles and you can't clean it off. It just gets everywhere," Midgley said.

"You can wash your vehicle one day and the next day it's back again."

Midgley said he had lived in the house for six years and while the problem had been ongoing it had got worse in recent months.

"It's probably only in the last six to eight months that I've really noticed it."

Another neighbour, who did not want to be named, said her house was being splattered with the substance all of the time.

"You clean your windows and then it's just back again. It's just really annoying," she said.

"It's really hard to get off."

She also wanted to know what was causing the mess.

"It would be nice to know exactly what it is."