Smith, who retired hurt on 80 then was trapped lbw without offering a shot on 92, was the central figure in a chaotic post-lunch session on day four of the second Ashes Test.

The former skipper was hit on the elbow, forearm and neck by express paceman Jofra Archer during a violent spell of short stuff.

The third and most brutal strike prompted the shocked crowd to fall silent as Smith slumped to the ground.

Smith was back on his feet soon after and keen to keep batting but left the field at the insistence of team doctor Richard Saw, who wanted to conduct a thorough concussion assessment in the rooms.

Broadcasters initially didn't show a replay of the nasty incident, such was the level of concern.

Fielders and teammate Pat Cummins rushed over to Smith after he hit the deck face first, with the scare serving as the latest reminder of Phillip Hughes' tragic death.

Ian Healy has slammed the "disgusting" behaviour of a small group of fans who booed Steve Smith after he was hit on the neck.

There was a standing ovation when Smith walked off the ground, and most members of the crowd also stood and applauded when he resumed his knock.

But some of England's supporters opted to boo the batsman, who has copped plenty of abuse and jeers during both the World Cup and Ashes in his comeback tour from a year-long suspension, at both junctures.

Healy, who is hosting Nine Network's coverage of the showpiece series, was shocked.

"It was disgusting," Healy said.

"Lord's won't be happy with that either. They don't like any 'yobbo' element of crowd behaviour."

Australia coach Justin Langer shrugged his shoulders then choose his words wisely when asked about the boos.

"What can I say? ... I've spoken enough about the boos, there's nothing we can do about the boos," Langer said.

"There is also a lot of people standing and applauding him."

The Barmy Army, who taunted Smith for his role in the Cape Town cheating scandal throughout the Edgbaston series opener, distanced itself from those fans booing the stricken superstar.

But paceman Chris Woakes opted against condemning the noise.

"People are entitled to do as they wish, that's up to them and what they feel is necessary," Woakes said.

"What they think as a person is right and wrong. For Steve to come out, having been hit the way he was, does show some courage and character."

Mark Taylor, who is on deck at the home of cricket as a Nine pundit, lashed the booing as "very disappointing".

Shane Warne suggested booing Smith in other instances was one thing but to add insult to injury was not on.

"Just get up, clap and say that's courageous, very brave," Warne said in commentary.

Langer said, "You never like seeing your players get hit like that."

"An ugly incident. Thankfully Steve has come through it OK.

"There's obviously some pretty rough memories of a blow like that.

"I'm sure he'll be very sore tomorrow - his arm and his neck - but he was in good spirits."

David Warner and other members of Australia's squad were at the SCG in 2014, when Hughes was tragically hit on the neck in a Sheffield Shield match.

A battered Smith didn't field on day four, having rushed off for X-rays that confirmed his arm wasn't broken, and will undergo further concussion tests but Langer expects the star will "play the game out".

Paceman Chris Woakes was fielding at fine leg when Smith was struck but immediately grasped the seriousness of the situation.

"You get a feel from the noise more than anything. You could hear it was fleshy," Woakes said.

"When that's the case, you worry."

Smith remains key to Australia's hopes of enhancing a 1-0 series lead but the overriding sentiment in the visiting dressing room is relief.

Langer, who famously wanted to bat in a Test against South Africa in 2006 despite being hospitalised with concussion, revealed just how eager Smith was to get back out there after passing his medical tests.

"These are like my sons alright, so you're never going to put them in harm's way," Langer said.

"He was going 'mate, I just want to get out there. I can't get up on the honours board unless I'm out batting'.

"I asked him behind closed doors two or three times. I asked him in front of the group, he just kept going 'all good, all good coach. I'm ready to go'.

"What else do you do? The medicos cleared him ... all that he was worried about was [the arm injury]."

A recent rule change means that concussed players can be replaced mid-Test, subject to the match referee's approval.