If you are working on the North Shore, why commute to the South? Houses, schools, hospitals, etc are all just the same on the Shore as across the Nippon Clipon...
As you are moving to Devenport, I would guess you are with the NZDF - find a nice house to rent for a few months to get your bearings before diving into buying and setting permanent roots.
The suburbs in ferry commutable area to Devenport are some of the most expensive in Auckland, you can get fantastic value for your pound on the Shore.