Greater Western Sydney have staged a stunning comeback to beat Melbourne by 45 points in Canberra, marking their best-ever start to an AFL season.

Down 27 points at halftime, the Giants booted nine goals to none in the third term to win 15.11 (101) to 8.8 (56) at Star Track Oval on Saturday.

It was the highest-scoring quarter in their short history and second straight win of the season following last week’s round-one triumph over St Kilda.

Until now, the club hadn’t won their opening two games since joining the competition in 2012.

Key forward Cam McCarthy and Dylan Shiel booted three goals each, while Stephen Coniglio (26 disposals) and Devon Smith collected two apiece.

Ruckman Shane Mumford also nabbed a major in arguably his best performance for the club.

The Giants started with a spring in their step, but it was the visitors who set the pace early on.

After a scoreless opening 10 minutes, Melbourne booted two goals within 90 seconds through Tom McDonald and Dean Kent.

The momentum seemed to go entirely the Demons’ way after that, their fast ball movement making the Giants look sluggish for a 24-point lead at the first change.

While somewhat composed in defence, GWS’s forward halves appeared to struggle to piece together some valuable plays.

It wasn’t until five minutes into the second quarter that Shiel got the Giants their first goal. It took another 24 minutes for goal No.2 via McCarthy to go into the main break 27 points down.

But GWS came out firing in the second half, stringing together 13 unanswered goals on the back of a dominant ruck performance by Mumford.

Demons only managed a single major in the second half through Ben Newton, who’d earlier been subbed out for a suspected concussion.