Maritime NZ is investigating a collision between two jet boats on the Whanganui River.
The formal investigation comes after inquiries were made earlier this week into the incident.
Four people were hospitalised after last week’s crash, with one female seriously injured.
The crash was on a stretch of river about 500m from Hipango Park about 1pm on March 1.
Maritime NZ said it had spoken to witnesses as well as local emergency service responders.