You know how twice a year clocks go backwards or forwards depending on how Daylight Saving goes? And how fire services around the world remind people to change their fire alarm batteries on those dates, and test their alarms afterwards?
In Christchurch there is an addition to that: tsunami warning sirens.
Twice a year sirens sound up and down the coast (I think there's 47 in total) in the bi-annual tsunami warning test - at the start and the end of Daylight Savings Time, 11 am - and just like when fire alarms get tested, it enables people to know that sirens work and that when they hear that noise for real, to run inland.
So... yeah: welcome to New Zealand.