England were reeling a fortnight ago after being smashed 36-0 by South Africa.
It left them needing to beat Samoa and Tonga to seal for a quarter-final place, and England were back at their Versailles base today having accomplished their mission.
The Wallabies now await in Marseille, which will be the biggest Test match for both countries since Jonny Wilkinson's drop-goal saw England crowned world champions in Sydney four years ago.
Lewsey, who started that game at full-back, said: "We let ourselves down enormously against South Africa but we have drawn a line under that and we've worked and been positive.
"Of course (Australia) are favourites, but our concentration is on ourselves.
"We know what they are capable of, and people probably haven't seen what we are capable of. Perhaps we don't know ourselves yet, because we haven't delivered it.
"But if we can keep building in the process that we are doing, then we will give ourselves a chance."
England trailed the Tongans 10-3 before wing Paul Sackey's try double featured in a purple patch of 16 unanswered points that ultimately built a victory platform.
Lewsey added: "I was delighted to win, and then clapping the crowd afterwards - all positive stuff.
"But without sounding like a sour puss, it is time to put a lid on it.
"It was Tonga this week and next week it is Australia. That is some serious stuff, and an entirely different kettle of fish."
While England's reign as world champions could end in seven days' time, Wasps speedster Sackey has already proved himself one of the squad's major success stories.
The 27-year-old's second successive World Cup brace has taken him to five touchdowns in just seven Tests, transferring form from the domestic stage that has seen him amass more than 50 tries.
Sackey said: "The Tonga game was the biggest game I've ever played in, and it is obviously satisfying to score two tries.
"For the second try, I had to show a bit of pace, and I knew I had the gas on him (the Tongan defender), so I just put my head back.
"I think there is more to come from the squad. We have still got a lot to work on - it wasn't our best performance.
"It was probably about the same physicality as the Samoa game, and we knew what we had to do."