About 1500 litres of aviation fuel leaked into Lyttelton Harbour after a storage tank was damaged by a landslip during the storm that hit the area earlier this week.
The immediate priority was stopping more fuel going into the harbour, Environment Canterbury said today.
A boom had been deployed in the harbour and stormwater outlet to try to collect the fuel.
Assessments of the water by Environment Canterbury marine response staff yesterday found small amounts of fuel were seen in several locations - but it was very thin and appeared to be dispersing naturally.
Further monitoring would be undertaken today, Environment Canterbury said.