Joe Root's men mustered a solitary wicket on a grim third day at the WACA. Smith (229no) celebrated two centuries and fifth-wicket partner Marsh (181no) threatened to turn his maiden hundred into a double too on his home ground.
There was next to nothing, armed with new ball or old, that England's bowlers could do to even slow the process as Australia powered to 549 for four and an already ominous lead of 146.
An unfamiliar forecast in these parts promises disruption from rain over the final two days, but England will still have to excel themselves to avoid going 3-0 down in the series and losing the urn before Christmas.
Smith ensured one-way traffic almost throughout in his career-best innings.
He needed just five overs of a sunny morning to complete his century, so he could dedicate the remainder of the day to inflicting a second instalment.
Shaun Marsh's departure was an unexpected blip for the hosts, Moeen Ali responding to two leg-side fours for the left-hander by producing a perfectly-pitched off-break which took the outside-edge to slip.
Any raised English hopes were cruelly fleeting as Shaun's younger brother quickly announced himself in imperious form and full of determination.
It was a combination which allowed him to dominate even Smith in an unbroken stand of 301 as England became not only bereft of a wicket-taking threat but a method to arrest the scoring rate too.
Their plans appeared to lurch from one extreme to another, telegraphed by field placings.
Marsh was soon under way with a clutch of crunching drives, so England responded by going short to both batsmen after lunch to a leg-side field and no one in front of square on the off.
Still Smith found the gaps, and Marsh demonstrated he was at least as comfortable on the back foot.
He completed his century with a square-drive for his 17th four off Stuart Broad in the over before tea, sparking not only his own richly-deserved celebrations but that of his majority support all round the WACA.
Craig Overton, nursing a cracked rib after falling awkwardly as he tried to take a return catch the previous afternoon, found it especially tough going to keep Smith and Marsh quiet. James Anderson, arguably, posed the most credible threat.
If so it was purely relative, though, and no one even came close to breaking the epic partnership.
The Australia captain passed his 200 with 26 fours and a six from 301 balls, spread over more than seven-and-a-half hours, and Marsh was closing on the same milestone by the time stumps were drawn to allow the tourists much-needed respite at last.