Sealink

I have used Sealink on many occasions for travel to the outer islands of the Gulf, namely Hauraki and further to Great Barrier Island.
The ease at which this operation is carried out is second to none with turning up in your car and driving onto the ferry as hard as it gets.
As a frequent car ferry user travelling to Waiheke, the newer ferry is a dream, a drastic improvement from the older ferries — some of which are still in use. The newer ferry is thankfully faster (by 15mins I think) and offers a lavish lounge and café if chilling out in your car is not for you.
There is no extra administration after buying a ticket which makes this trips as hassle free as possible.
On a previous trip to Great Barrier by car ferry, one person in the group had managed to misread a midnight time on the group tickets meaning had missed our ferry. We were squeezed onto the freight ferry and shipped back the next day, at no extra cost which was a godsend.