Wikipedia says Rena was New Zealand's worst maritime environmental disaster. Its overall worst maritime disaster was of course the 2013 America's Cup: http://en.wikipedia.org/wiki/2013_America%27s_Cup
Still too soon, too raw! :) Hopefully the Brits get a team in next time around so I have 2 teams in the running.
The Wahine, while not the worst maritime disaster NZ has ever had is one of the more recent [0].
This video [1] showing a ferry bouncing along gives you a good idea of how rough it can get on the route between Wellington and Picton. I had friends on there one day when several people were hospitalised with broken bones!
[0] http://www.nzhistory.net.nz/culture/wahine-disaster [1] https://www.youtube.com/watch?v=aUWascqCBe0
The container ship was carrying oil which spilled into the water causing a lot of distress. Very small "disaster" compared to others in the world like it, but nevertheless, a really stupid human error which led to the shipwrecking.
[1] http://en.wikipedia.org/wiki/Rena_oil_spill