The legality depends on which state you are in. In NSW, it's covered by the Surveillance Devices Act 2007 (NSW). Link
Basically, use of an optical surveillance device owned by you, on your property or your vehicle is not restricted in NSW.
In Vic, this falls under the Surveillance Devices Act 1999 (Vic). Link
Video surveillance of activity happening outdoors is not restricted under Victorian law.
The Victorian Law Reform Commission recently compiled a report which discussed use of public place surveillance in Vic, it's a good read.
http://www.lawreform.vic.gov.au/wps/wcm/co...+-+final+report