Smith’s form dipped slightly between June 2023 and December 2024 when he went 23 innings without a Test hundred, but he made two against India last winter and hit 141 and 131 against Sri Lanka at the start of 2025.
After a two-month break, he began his build-up to this year’s Ashes in late October and scored 118 in his first innings.
“To win the series, we have to keep him [his average] to under 50, don’t we?” Broad said.
“He’s gonna score a hundred, it’s what he does.
“[If] he averages 40, that’s at least 15 under his career average, so you’re doing brilliantly. I think you have to get him early.
“If England aren’t celebrating in his first 40 runs, it’s going to be a long series.”
England have had various plans to counter Smith’s unconventional technique over 14 years of Ashes series.
Broad spoke of developing a new outswinger for Smith before the 2023 series while England have targeted Smith’s pads, bowled wide of off stump and even bowled for catches at leg gully in 2019 – a series Smith began with twin hundreds at Edgbaston.
“I remember Jason Roy was fielding there at leg gully, and he was diving left and right for the first 30 balls, just missing him there, just missing him there,” Broad said.
“Smith got away, got two hundreds in that game, unbelievable innings.”
Broad added: “I remember sitting in a bowlers’ meeting about Steve Smith in 2017-18 and they were like ‘oh, you could try and attack his stumps early’.
“‘Oh yeah, can we just click on how many times he’s been bowled in Australia in the last five years?’ Once.”
The upcoming series, which begins in Perth on Friday (02:30 GMT), looks set to see Smith face Jofra Archer for the first time in Tests since 2019, when he was famously unsettled and struck by a bouncer during the England paceman’s fierce spell at Lord’s.
Smith has always been keen to point out Archer did not dismiss him in that Test – he had to retire hurt and was out lbw to Chris Woakes on his return – though Archer did have the right-hander caught on the pull in a one-day international in 2024.
“Can Jofra Archer be that guy?” Broad said. “Nicking him off from back of a length, just chasing it.
“Because Jofra Archer has that angle of nipping back towards the stumps, can he nip one away? Smith’s looking for the nip-backer.”