The Law is a confusing thing and open to interpretation.
The Laws you've put up such as Section 8 of the Surveillance Devices Act doesn't really regulate the use of video surveillance directly, but rather does so indirectly through regulating access to premises or interference with property to install, use or maintain the device. The consent that is referred to is actually consent to the entry onto (or interference with) the premises or vehicle for the purpose of installing, using or maintaining the device, not consent to the installing, maintaining or using per se. So if its your property you can install to your hearts content. So as a result, you as the property owner/occupier whose property the device is installed that must consent (rather than people that might be observed) so the question of whether entering an area to instal a CCTV is moot.
Section 12 does cover video surveillance records, however it only applies if the record was obtained in contravention of the Act (which mainly means contravention of section 8 ), which as above if its your property you have the right to install CCTV systems on your property for the surveillance of your property. You can't point a camera into another's property or even if it partially looks over it (there are laws on such thongs including voyeurism etc laws that are very harsh etc. You can also get an AVO bought against you based on threatening or intimidating behaviour). However a public areas such as a street or nature strip outside your house is a "grey area" when it comes to CCTV if it is included in the background and primary purpose of the system installed is your property. Of course any installation will be subject to the Law, however that doesn't mean you can't so it on your own property.
If I was to do it (and I have), make sure that the "primary" focus of the camera(s) is your property. Any installation must be done with due care. Point it away from the neighbour's or private property and keep the camera primarily focused on your property. If it just so happens to view a public area such as a street in the background then whilst not ideal, it gives you a "primary purpose" argument. The sound thing is a simple no go. You can't legally surveil or record and keep/listen to a persons conversations without express permission or legal order from the courts (such as police warrant etc).
Either way if nothing else you will be able to see whose doing it and go from there.