Marnus Labuschagne will undergo scans on an injured finger and Steve Smith was left feeling worse for wear after a bruising third day of action against Pakistan in the first Test in Perth.

Australia went to stumps on Saturday at 2-84 - an overall lead of 300 - after Pakistan were dismissed for 271 in front of 15,956 fans at Optus Stadium.

Smith (43no) wore a series of body blows at the hands of Shaheen Afridi late in the day, including one on his left arm that required several minutes of treatment.

But there is more concern surrounding Labuschagne, who injured the pinkie on his right hand after a Khurram Shahzad delivery jagged back and struck him on his bottom hand.

Labuschagne was able to bat on in discomfort, but he was dismissed a short time later for two.

He was sent off for scans on Saturday night.

"He was just with the doc and physio for the last hour just chatting through it and doing a few tests on the finger," Australian paceman Josh Hazlewood said after play.

"(It's) pretty sore I think."

If scans show the finger is broken, Labuschagne is at risk of missing the Boxing Day Test at the MCG.

On a day of storylines, Nathan Lyon claimed the scalps of Imam-ul-Haq and Aamir Jamal to move to 499 Test wickets, putting him on the verge of joining the late Shane Warne (708 wickets) and Glenn McGrath (563) as Australians to have cracked the magical 500-wicket mark.

With the crowd urging him on, Lyon had a few "nearly" moments against Salman Agha (28no off 76 balls) and No.11 Shaheen (4).

But wicket No.500 will have to wait for another day after Travis Head came on and tempted Shaheen into a wild shot that was easily caught.

Lyon ended the innings with 3-66 from his 24 overs, while Pat Cummins (2-35) and Mitchell Starc (2-68) also took multiple wickets.

After posting 487 in their first innings, Australia decided not to enforce the follow-on, but their second innings started in disastrous fashion.

David Warner, on the back of 164 in the first innings, fell for a fifth-ball duck when he skied Shahzad with a mistimed pull shot.

Labuschagne's departure reduced Australia to 2-5 in the eighth over.

Smith had to deal with some bodyline-style bowling after that, with the former skipper copping a rising Shaheen delivery on the shoulder before sending the next ball for six over fine leg.

A short time later Smith wore another Shaheen delivery on his arm, causing a delay of several minutes while the 34-year-old was assessed.

Smith made it to stumps alongside Usman Khawaja (34no off 106 balls).

"It's all right, a bit of a bruise, but I'll live," Smith told Fox Cricket after play.

"That was challenging there. The light got a bit dark, and Shaheen's got a whippy action with his wrist, so it was quite hard picking up the length of those, but I got through it."

The tourists showed promise with the bat early on Saturday, but they lost 3-14 during a calamitous period before lunch.

Hometown hero Mitch Marsh began the carnage, removing Babar Azam for 21 when the Pakistan star edged an outswinger to wicketkeeper Alex Carey.

Australia's position of dominance was assured when Lyon also struck, dismissing Imam for his 498th Test wicket.

Imam had faced 199 balls for his 62, but lost concentration as he charged down the wicket and missed Lyon's delivery, gifting Carey an easy stumping.

Another sharp piece of work by Carey - this time to stump Aamir Jamal - gave Lyon his 499th Test scalp, but Head came on to snare the last wicket of the innings.

MOST TEST WICKETS BY AN AUSTRALIAN

708 - Shane Warne

563 - Glenn McGrath

499 - Nathan Lyon*

355 - Dennis Lillee

335 - Mitchell Starc*

313 - Mitchell Johnson

(* denotes: still playing)