One of my favourite wild places to go near Christchurch is Birdlings Flat. It is very wild beach with a significant undertow and you can't swim there. But it is wonderfully scenic in a bleak way. A bit like Chesil Beach in Dorset.
Te Roto o Wairewa/Lake Forsyth is just beside the beach. It has been dangerous for years, but for a different reason. Nitrate run-off from agricultural land that feeds into the waterways that's ending the lake cause toxic algal blooms. It's too dangerous to even walk your dog your dog there.
The saddest thing is that this isn't the only lake or river that is in this condition. All over New Zealand the pressure of intensive agriculture is ruining the waterways, making rivers that kids played in a generation ago unswimmable now.. Some farmers are doing something to stop it (by fencing stock out of rivers, planting at the edges of waterways to soak up any run-off and reducing stock and chemical levels to prevent excess nitrates reaching the water). Unfortunately a lot of farmers are either ignorant of what they need to do or more concerned about profit.
Update: we walked up the Opuha river in Canterbury. A dry summer has been compounded by intensive agriculture and over-extraction of water. The river is very low and to add insult to injury we saw cows walking in and out of it and every feeder stream we crossed was full of cowshit.
Update: Shortly after I posted this, a report highlighted the trouble the NZ environment is in: https://www.theguardian.com/world/2019/apr/18/decades-of-denial-major-report-finds-new-zealands-environment-is-in-serious-trouble