Actually in Australia, it is indeed considered an invasion of privacy if you take a photograph of someone on private property EVEN if you are on public land. It's a simple as that, the same in the UK, Hong Kong, Mainland China, the USA and every EU country. If they were outdoors, say in a front garden, no judge will give it the time of day, but shooting through a window - into a building is a completely different kettle of fish.
I would be really careful if I were you, especially those of you with minors in your pictures, software exists which enables you to search the internet for your face in pictures, I use it daily in my job. It's getting quite good and I bet google will update their picture search to include this functionality in the near future.