This was in the news a few days ago. I believe the cause is a very strong La Nina condition - sea surface temperatures in the E Pacific equatorial region are actually lower than normal.
This is the current map from Earth.nullschool.net.
Water temperatures at our local beaches in Chch are now about 5 deg C above average for this time of year. It's very noticeable - I was swimming in NSW a couple of weeks ago, and due to easterly conditions, it was actually colder swimming at Bondi ("only" about 17 deg) than here (now about 20 deg C - which is almost tepid - well maybe not quite - but very pleasant for a swim). Combined with NE to NW conditions with temperatures in the high 20s to low 30s over the past couple of weeks, it feels like I'm in the wrong country.
I've got no idea if this is going to persist and for how long - and I'm sure that I've never experienced anything like this weather before at this time of year. Have been running the AC at home a lot - not looking forward to seeing the power bill.