Did Prime Minister Robert Menzies believe in the domino theory?

Very much so. In 1966, Menzies stated: "I subscribe to the domino theory ... because I believe it obvious ... that if the Vietnam War ends with some compromise that denies South Vietnam a real and protected independence, Laos and Cambodia, Thailand, Malaysia, Singapore and Indonesia will be vulnerable ... this domino theory ... has formidable reality to Australians who see the boundaries of aggressive communism coming closer and closer.'