A 40-year-old Auckland man has been decapitated in front of family members while testing a kitset hovercraft he built, police say.



He died on Muriwai beach on Sunday after being hit by the propeller he was checking, police crash investigator Sergeant Colin Nuttall said.



The man's partner was believed to have seen it but she is yet to give a statement.



Nuttall said they were still unsure of what had happened and they had called in an engineer to examine the machine.



He said they believed it was the first time the man had used the machine.



Conditions had been fine and calm at the beach.



"We will be talking with the family; some of them were present on the beach."



It was not known whether the man had experience with hovercrafts and where he had purchased the kitset. Police would report to the coroner.



"At the moment there are no regulations. You cannot register hovercraft on the road and they do not have warrant of fitness. So you can build them and operate them off the road without rules,'' Nuttall said.



He said Maritime New Zealand was also involved in the investigation.



Police have not released the name of the victim.