If you need to get between airports, many of us would probably fly or get an inter-city bus. Another option is to do a campervan return. There are certain cases when this works well:
1. Moving home - If you have more luggage than you'd be taking on a flight, then you can take that with you, i.e. Say because you are moving luggage to a storage facility on your way to the airport.
2. Different departure - Say you live in one majority city or town like Queenstown, but you have an international departure from Christchurch or Auckland.
3. Expensive domestic flights - Say you live in NZ where domestic flights can be just as expensive as your international flights.
4. No bus service - Say you left your bus booking too late, then campervan 'returns' can be a cheap way of getting to the airport. You also have the option of making a slight holiday out of the trip.
5. No car storage - I will be flying from Christchurch airport in June'12 and will probably looking to sell my car at that time. I will probably look at buying a new car when I return rather than keep one in storage for 3 months over winter. In this case, it makes sense to sell my car in Queenstown, a popular entry point, and to return a campervan.
This is of course just another option for those probably with a lot of luggage; or wanting to do some shopping, etc in another city. In most cases, you will probably just get that domestic flight. The trick is to know where to go to get your campervan 'return' and remembering. There is a litany of campervans needing redistribution at Queenstown airport. You drive past them on the way to New World shopping centre. Try Camper Returns.
You can get the vehicles for around $5/day, with a $100-300 fuel allowance depending on the distance and their urgency. There are time constraints, however you have the flexibility to pay extra for extra days.