Tel: 64 9 356 7272
http://www.holidaycity.com/sail-auckland/index.htm
I booked via Holiday City (linked above) and the process went smoothly.
When I stayed here last year, the hotel was named Sail City, and I believe that had been a recent name change. I have no idea if they are guest-friendly as I didn't try to bring a girl back during my three day stay, but it looked like it would probably be ok if it was discreet.
Rainbow Hotel is a 3 star hotel that has an average rating of 3.5/5 stars from 95 reviewers. Location seemed perfect, directly across the street from IMHO the best brothel in Auckland and centrally located from the rest.
The hotel was Asian-run and nothing fancy. They had several computers downstairs, but dialup connection was a bit spotty. A 24 hour internet cafe is a few blocks away, but they didn't allow for laptop connections. I had to go about 6 blocks to find one that did. A 24 hour Denny's is down the block for late night snacking.
The rate I paid last year was 65 NZD or about $41 dollars per night.
I arranged transport back to the airport at the front desk via shuttle for 22 NZD ($14 USD).
I'd stay here again, although personally I'd probably prefer a hotel with broadband access or closer to a cafe where I could hookup my laptop.