Wellington Pier dive platform
The story goes that kids kept jumping off the ambulance kiosk. This being Wellington, the city decided to build them an official platform. Notice that it's almost the same height as the building and much easier to get up to.
No belly flops allowed, anything else goes.
That is, if you were actually allowed to swim in the water. First time I've seen this sign, so it might be temporary. btw, see those kids on the far left. Yeah they're jumping into the other opening in the pier. I guess the sign doesn't apply to water that's 10m away.