England were dismissed for 260 after winning the toss, before Australia to edged to 0-25 from seven overs before stumps.
Alastair Cook (67) and Ian Bell steadied the innings with a 72-run partnership for the fifth wicket, only for Siddle to again intervene after tea.
England slumped from 4-197 to 7-197 in three deliveries as Siddle had Cook caught at first slip by Shane Watson, Matt Prior clean-bowled and Stuart Broad LBW following a video review.
Siddle said it was a moment he would cherish and thanked the crowd of 35,339 for giving him a lift at the top of his bowling mark for his hat-trick delivery.
“It was weird, just the cheering and that,” he said.
“I don’t think I’ve ever heard that in Australia before.
“To have that behind you and pump you up a bit and the way the ball came out, it was good.
“I’d like to say it was exactly what I wanted. I was looking to hit the top of off (stump).
“It got him (Broad) on the full so it was good.”
The fired-up Siddle, a former junior wood-chopping competitor from country Victoria, powered his way to his sixth wicket when Graeme Swann was trapped LBW for 10 at 8-228.
Debutant left-arm spinner Xavier Doherty (2-41) had a smile to match Siddle’s after claiming top-scorer Ian Bell for 76 and James Anderson for 11.
Earlier, England suffered another case of first-over jitters after skipper Andrew Strauss lasted only three balls on a green-looking pitch.
The left-handed opener was caught at gully for a duck from the bowling of Ben Hilfenhaus.