The Junior All Blacks scored two tries in the first 10 minutes to lead 14-0.
Down 26-0, things got worse for the Australians when both Jayden Blake and Mason Gordon were sent off within minutes of each other, allowing New Zealand to score two more tries immediately after each infringement, ending the half 40-0.
The onslaught continued in the second half though the Australians did manage to get on the scoresheet.
Australia finished third after Argentina's victory against Fiji.
