The injured man has been taken by Westpac Rescue Chopper to Auckland Hospital in a serious condition. The Waitemata Police Serious Crash Unit is investigating.
"Whether alcohol was involved will be part of our investigation. The vehicle was not road-legal and we've seized it so it can be mechanically examined," said Inspector Mark Fergus, Road Policing Manager, Waitemata Police.
"An investigation is also underway into who actually owns the mechanical bar stool."