If you haven't checked the <www.rivers.org.nz> website, that's a good starting point.
For Kiwi slang, get a copy of The Godzone Dictionary by Max Cryer.
ChCh is nice (yeah, right) but before heading for the west coast, take a bounce to Lyttelton, the old port town (Rat & Roach for burgers & pints, Volcano Cafe for dinners, Harbour Light for music, Wunderbar for booze-ups). There's a backpackers there (Kiwi for budget hotel). Fun just to walk around. Harbour cruises & Quail Island walks.
If there's a dry spell on the west coast, go paddle Okarito Lagoon, a huge shallow estuary with incredible birds and gorgeous clear moraine creeks that flow through native bush, white pine and rimu. There should be cruising 'yaks for rent. Do ask about tides.
Pretty hard to have a bad time there, so take it easy.