Investigators are looking to piece together the circumstances surrounding a fatal boating accident at the entrance to Auckland's Manukau Harbour

The Transport Accident Investigation Commission has opened an inquiry into Saturday's accident which claimed the lives of three men.

A fourth man remains in hospital in a moderate condition.

They're appealing for witnesses who may have seen, photographed or videoed what happened.

Chief Investigator of Accidents Harald Hendel says they're keen to hear from people who may have noticed anything about the boat, or who've found anything washed up.

He says at this stage they don't know what type of vessel was involved, and witnesses could help with this.