The Ashes series may be reduced to dead rubbers, but Alastair Cook and Stuart Broad proved their England careers remain very much alive on Wednesday as they helped Joe Root's team dominate Australia on day two at the Melbourne Cricket Ground.

Veteran paceman Broad finished with 4-51 in another welcome return to form, hastening Australia's first innings dismissal for 327 shortly after lunch.

There was also satisfaction for the seasoned duo in responding to their critics, some of whom demanded they be axed midway through the series.

Former England captain Michael Vaughan was among them and said Broad should have been dropped for Melbourne after he went wicketless for 142 runs in Perth, his worst ever Test figures.

Not that Broad noticed as he bunkered down in the leadup, searching for the killer instinct that had deserted him for much of the series.

"It was only two weeks ago that I was on holiday with (Vaughan), so I don't know what he's been saying," Broad told reporters with a smile.

"To be honest I've had one of those weeks where you get your tin hat on and duck down and don't really see much, so I've been very unaware about things being written and things being said.

"You've got to say your opinion, you've got to be critical at times and I deserve criticism after the Perth Test defeat for sure.

"I am not going to hold any grudges or get too disappointed if people slag me off because at the end of the day in 15 years' time I might be doing the same. Not six months."

Former captain Cook was already under the microscope before arriving Down Under, with doubts swirling about his motivation to carry on for the long haul.

He struggled through 10 innings without a half-century before his breakthrough on Wednesday, his highest Test score in Australia since a knock of 189 in the final match of the 2010-11 series in Sydney.

A stroke of luck helped him on his way, with Australia captain Steve Smith dropping him on 66 when he nicked all-rounder Mitchell Marsh to slip.

Two days after Cook's 33rd birthday on Christmas Day, Smith then gift-wrapped the opener's century by bowling a profligate final over before stumps.