Initially, the Australian government sent about 30 military advisors to help train the RVN army and when combatants were sent in 1965, Menzies claimed that South Vietnam had 'invitied' Australia to help them fight.
The Australian government was very aware of Communism spreading throughout Asia and labelled this is as the 'domino effect', fearing the Australia would ultimately fall to communism because of our geographical proximity to these countries. As a result, Menzies promoted the Forward Defence strategy to fight communism before it came to Australia.
Menzies also implemented the Forward Defence strategy as a means of preventing war from coming to Australia. He publicised the view that "... The takeover of South Vietnam would be a direct military threat to Australia...", fearing that the takeover of South Vietnam would mean war for Australia.
Fiurthermore, Australia had a number of political obligations to fulfill and denying their responsibilities as part of the ANZUS, 1951 and SEATO, 1954 treaties would label Australia as cowardly, unreliable, undependable and untrustworthy. It would have also exposed Australia to more military threats and left us without support from key allies.
Finally, Australia had joined the Vietnam War as a demonstration of support for our American ally. After the Second World War, it had become clear that America was the new superpower and the Australian government felt they needed to have the world's most powerful nation on their side should Australia be attacked.