To add to Ponala's reply, it's important to note that the definition of "private activity" in the Surveillance Devices Act explicitly excludes:
1. "an activity carried on outside a building;" and
2. "an activity carried on in any circumstances in which the parties to it ought reasonably to expect that it may be observed by someone else."
In regard to #1 - You can't do anything about a camera that looks into your yard - that's perfectly legal.
In regard to #2 - If you have blinds and they are closed, the camera can't see inside and you have nothing to worry about. If they're open, or you don't have blinds or something else to block the view, then the "circumstances" are different and anyone carrying on any activity inside "ought reasonably to expect that it may be observed by someone else." - provided that other people may have a view of the window.
This is all pretty easy when it comes to front and/or side windows that can usually be seen from the street. But back windows? Well the principle is still the same. If you look out your own windows, you can likely see windows on surrounding houses. If those windows don't have blinds or whatever, then you can likely see inside. The same works in reverse - if you can see into their windows, then it is always reasonable to expect that they can see into your window as well.
In reality, this issue has always existed - long before cameras were ever invented. Any person who walks down a street can look at a window and there is nothing anyone can do to stop that - and that always has been and always will be the case. Therefore, if you want to keep your activities private, the onus always has been and always will be on you to take reasonable steps to ensure that people can't see inside. As the old saying goes, "if you want privacy - don't advertise."
So the only question is: "Does the camera provide someone with a view into your window, where without the camera, noone would have a view into that window?"
If the answer is "yes", then the installation and use of the camera could be in breach of the Surveillance Devices Act** - and that is the question that everyone wants tested in the courts.
If the answer is "no", then it likely wouldn't be a breach. This is because if people already have a view into the window, it would be next to impossible to successfully argue that the existence of the camera makes any difference to your privacy.
** My opinion is that if a camera provides a view into a window that no natural person could already have, then it is definately a breach of the Act. This comes from the definition of "private activity" and the second exclusion as stated above, which uses the words "may be observed by someone else". In my opinion, in this context, the words "someone else" relates to a natural person only and therefore, does not include cameras. If you do include cameras in this exclusion, then you effectively allow any person to point a camera directly into someone elses window, thereby negating all rights to privacy. It is not the intention of the legislation to do that, thus the exclusion should be interpreted in such a way that it applies to being observed by a natural person only.