About 82,000 will be at Twickenham on Saturday, and defeat means England would be the first World Cup hosts to fail to reach the knockout stages.
They have won four of the past five meetings with Australia.
"It is a huge game, at home. With the whole country behind us, we'll be difficult to beat," Lancaster said.
Australia have won both their matches in the tournament, while England let slip a 10-point lead with 30 minutes remaining to lose 28-25 to Wales in their previous encounter.