Scotland impressed in the 10 overs that were possible in their opening game against England and comfortably beat Namibia.
They then thrashed Oman to put themselves in pole position to qualify, although England hammered the same opposition on Thursday to go ahead on net run-rate.
There are unlikely scenarios where Scotland can go through even if they lose and England win, but Leask said Scotland are treating the game as a “must-win”.
“We knew coming into this game it would be,” he said.
“England had two tough games to play and they’ve played the first one very well.
“We always knew this was going to be a must-win game for us and it’s a hell of an opportunity for us to go and play really good cricket and take on one of the best in the world.”
Former captain Kyle Coetzer believes it would be Scottish cricket’s greatest achievement if they were to progress.
Earlier this week, Australia bowler Josh Hazlewood discussed manipulating the game in an attempt to help Scotland’s net run-rate and knock out England, but team-mate Pat Cummins has said the idea was not considered.
Leask, captain Richie Berrington, openers George Munsey and Michael Jones, plus number three Brandon McMullen, have all made important contributions with the bat in Scotland’s three previous games.
“Our boys are firing, we’ve got some good skill sets, some nice spinners, some good seamers and then batters who are playing really well,” Leask said.
“We’re going at this game quietly confident. Our batters are probably going to have to step up knowing we’re facing the best in the world.
“They’ve probably got the best all-round bowling attack in the world. It’s going to have to be two or three batters in that batting innings that are really going to have to put Australia under the pump with a ball.
“We know that it’s capable of happening and that’s possible.”