Updated 4.59pm: A team of experts is at Foxton Beach assessing an oil spill.

It's not clear where the oil came from.

It's not clear exactly how big the spill is - but it's thought to spread along the coast to Himatangi beach, about nine kilometres north.

Horizons Regional Council spokeswoman Ally Koehler said an incidence response team is working out the best way to clean up the oil.

"We just ask that people stay away from the beach at this time as we clean up the spill."

On-scene commander Ian Lowe said clean up crews are limited by the tide, and the impending dusk.

He said they'll focus on cleaning up large spots of oil tonight, and start again in the morning when the tide is low.

Ian Lowe said any risk to marine life is minimal.

The source of the oil is still unknown.

Maritime New Zealand has been told of the spill, and is ready to help if needed.