Vilimoni Botitu opened the scoring with a third minute try before Aminiasi Tuimaba bagged a hat-trick to secure a 24-0 half-time buffer.
Joe Pincus pulled a try back for the Aussie 7s two minutes into the second spell before Fiji responded in kind through Josua Vakurunabili. Two tries in two minutes from Alasio Naduva sealed an emphatic victory as Gareth Baber's team made it back to back London titles and also confirmed their qualification for the Tokyo 2020 Olympics.
"Obviously we always plan to get out and do that but it doesn't sometimes go to plan. Obviously found it tough for the first four or five minutes there - Australia put some in. We knew they're a good side, they're a tough side to play against and we needed to get some scores on the board and we did it in some style," Baber said
The Olympic champions had advanced to the Cup decider after topping their pool with wins against Kenya, France and Samoa, beating Ireland 33-24 in the quarter finals before edging current series leaders the USA 17-10 in the semis.
The Americans outclassed France 31-14 in the playoff for third place which meant Fiji needed to beat Australia in the final to go two points clear with one round to play in Paris this coming weekend.
Ratu Meli Derenalagi was named player of the Cup final and was joined by Jerry Tuwai in the London Sevens "Dream Team", while Tuimaba was named the impact player of the tournament.
Meanwhile Samoa drop to seventh in the World Series standings after missing the Cup quarter finals for the first time in five tournaments.
After failing to win a match during the pool stage, the Manu 7s beat England 24-19, Argentina 14-5 and Scotland 26-17 on day two to finish in ninth place but were leapfrogged by Australia in the competition standings.