not sure when you are intending to visit but you should check out
wotif
it will give you the best deals - the langham is within walking distance of the museum and is very upmarket - has specials of $199 over easter weekend which is a bargain - the duxton is nice (in central city) and is $119 - if you prefer somewhere in Newmarket for shopping then the Quest is good ($145) and is self-catering which can help the budget
whatever hotel you choose if you are driving into auckland then get one where you can park at a reasonable rate (most of the city ones charge at least $10 per day)
good luck and hope you have an amazing anniversary!