All-rounder Mitchell Marsh scored a remarkable hundred as Australia built up a huge lead on the third day of the Ashes tour opener against Kent at the Spitfire Ground, St Lawrence.

The tourists were 322-4 at stumps in their second innings, a lead of 549 with a day to go.

And it was Marsh who stole the show with a scintillating display of power hitting after the tea break to reach his maiden century for Australia.

Marsh was eight not out at tea - 85 minutes later he retired with 101 alongside his name with the tourists 263-3 at that stage.

He scored his first 50 off 72 balls before he opened his shoulders and blasted his way to a second off just 20 balls with four sixes and four fours.

All up, Marsh, whose brother Shaun scored 114 in the first innings, hit five sixes and 12 fours in what was his third first-class hundred.

Marsh and fellow all-rounder Shane Watson added 153 for the third wicket after both had been promoted up the order, with Watson at three and Marsh at four.

Watson looked set to reach three figures too before he was caught at deep mid-wicket by substitute fielder Sam Weller off spinner Adam Riley for 81.

Riley finished the day with all three wickets to fall and figures of 3-114 off 19 overs.

He could have had a fourth wicket late in the day but captain Sam Northeast dropped Brad Haddin when he was on seven.

Haddin finished 11 not out while Mitchell Johnson was on 27.

Earlier, captain Michael Clarke promoted himself to open alongside Chris Rogers and the pair added 91 before Riley ended it.

He had Rogers caught at backward point by substitute fielder Weller for 45.

Clarke fell 19 runs later, and just three runs short of his second 50 of the match, after he holed out to Ben Harmison trying to hit Riley down the ground.

Both Clarke and Rogers hit eight fours in their stays at the crease.

The day started with Kent adding just 75 runs to their overnight total of 203-5 after being dismissed just before lunch.

Wicket-keeper Sam Billings didn't add to his 24 after being caught behind by Haddin off Harris with the first ball of the third over of the morning.

The other overnight batsmen, Adam Ball, combined with Claydon to take the score to 234 before Ball was dismissed for an impressive 45 after hooking Harris (2-51) straight to Siddle on the boundary.

Claydon, who was beaten four times by Harris with the first four balls he faced, was next to perish just three runs after Ball departed.

He was caught and bowled by Mitch Marsh (1-35) for 10 with the first ball of the day by the big all-rounder.

Riley was doing it tough, having his helmet broken by Siddle (1-46), and copping a barrage of short balls.

He battled on though, adding 18 with Hunn before Johnson returned to the attack and promptly rearranged his stumps with his first ball and Riley on nine.

Johnson finished with 4-56 off 18 hostile overs.

Hunn and Thomas shared a last-wicket partnership of 25, with both registering career best innings.

Hunn got the first runs of his first-class career, finishing unbeaten on 18, while Thomas made 13 before being caught by Johnson off leg-spinner Fawad Ahmed (2-82).