Foxton Beach resident Russell Ward said he was "dumbfounded" to find the oil along the beach.
"I didn't know what it was. It reminded me of little rocks until I touched it. It smelt disgusting, like crude oil. There is so much of it ... we have some of the cleanest beaches in the world here. In Vietnam, the beaches are filthy and it's common to find litter on them but not here," he said.
He warned people to not wear footwear inside after walking on the beach.
"Now I have some unwanted and unpleasant stains on my carpet."
Vicky and Charles Bowen were out walking their dog when they discovered the "smelly mess".
"I thought it was pebbles until I touched it. It's all along the beach. It's so strange, it's everywhere like shiny pebbles," Mrs Bowen said.
About 20 Horizons have walked the coastline to pick up oil by hand.
Horizons emergency manager and site commander Ian Lowe said they weren't sure about the cause of the contamination but Maritime New Zealand had been notified.
A sample of oil has been sent for testing."Maritime New Zealand are running some trajectory modelling.
However, the focus is very much on clean-up at this stage.
"People who find oil on any beach need to call Horizons immediately on 0508 800 800, he said.
- The Horowhenua Chronicle