Canterbury's main city is Christchurch, the garden city, which is where you will find the international airport for the region. Christchurch is located on the north side of banks peninsula, which is then surrounded by the vast Canterbury plains.
Canterbury picks up a lot of south swells that come up from the deep south, but these usually have to wrap around the bottom of the south island so are usually smaller the on the west coast and even then its only the exposed spots that can pick up the south swells. Canterbury does also get its share of north swells coming down the coast but these are less consistant and usual require the right weather patterns up in the pacific island area.
In Canterbury the water takes a while to warm up and is warmest between January to March, where you will still need a 3x2 steamer, spring suit or board shorts if your brave. In the winter it gets cold, you will need a 4x3 steamer or thicker, with booties and hood recommended.
When its flat there is still heaps to see and do from the attractions, events and shopping in Christchurch city. Canterbury is also home to a good choice of ski fields for the winter.