Officials are investigating the mass death of fish in a Kapiti river.

The Greater Wellington Regional Council said dead fish had been found in the mouth of the Otaki River.

It has collected carcasses and water for analysis, but said at this stage there is no obvious cause of death.

No sources of pollution or contamination have been found either.

The council will test the samples to determine if a natural marine ecotoxin may be responsible.

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In the meantime, the council is asking people not to fish, gather mahinga kai, or undertake recreational activities in the water in the estuary.

More information is expected to be released today.