Authorities hope all the oil which spilled from a boat in Timaru Port yesterday has been mopped up.
Between 40 and 50 litres of what is believed to be light fuel oil was discovered in the water about noon.
Environment Canterbury Regional Harbourmaster Jim Dilley said there had been no reports of marine life affected.
"There was tiny spots of sheen in a couple of locations and those were mopped up."
They would monitor the water for the next few days to ensure there was no oil that was missed, Mr Dilley said.
Mr Dilley declined to name the vessel believed to have been involved.
An investigation would get underway once the clean-up was complete.