Does anyone on south island do tours of north island?
Go see Richard at Richard's Adventures in front of Popeye's Hotel. He does a great tour of the Estuary on the North Side and I'm sure he'd be happy to show you around more of it.
Or you can ride your bike from the split in the morning. Its almost free. $15BZ round trip with bicycle. Lots of birdwatching. This is an alternative to the Sanctuary.
I went on this sanctuary tour and was amazed at the work they've done there -it's worth a look.