Is it ILLEGAL to download and view voyeuristic pornographic videos from websites. KNOWING that the subjects are 18+.
To be quite honest, I don't know much about Australia's law.
It is illegal for internet content providers within Australia to 'broadcast' internet pornography classified as MA15+ to R18+ unless such internet pornography is subject to an age verification system or internet pornography which may be classified as X18+ to RC content that is not subject to an Australian Communications and Media Authority (ACMA) infringement notice through exceptions.
However in most places, as long as it's not child pornography and you are the age of consent (depending on country) then it is legal.
As for voyeurism, they have published media of their activities online allowing others to view it. It's not illegal to spy on someone who consents to it.