My approach would be to take the following steps ASAP:
1. Contact the National Film and Sound Archive in Canberra (www.nfsa.gov.au) and tell them what you have;
2. Depending upon what the NFSA says, and upon whether you want to retain any sort of claim yourself to these films, speak to a private criminal lawyer who can help you in your dealings with the police;
3. Contact the police - probably the AFP in Canberra in the first instance rather than the state guys;
4. Don't post any footage or stills here or elsewhere on the internet until you have done the above - if the police decide this stuff is important to them, they are unlikely to have much of a sense of humour.