Triplefin has summed it up all very nicely indeed - (clever girl

).
If you are staying in Dorkland, then Goat Island is only about one hour away and you could slip out get a dive in and be back before you are missed
The following website should provide some info as well
Dive New Zealand, Dive News, views and articles, Dive New Zealand Magazine, Subscriptions
If rugby is your thing then March is during the Super 14 season - maybe worth taking in a game.
Snash