Ruthless Australia smashed four unanswered goals past Oman on Tuesday to reach the Asian Cup quarter-finals with room to spare and also ensure a berth for South Korea.

Goals from Matt McKay and Robbie Kruse and a Mark Milligan penalty put Australia 3-0 up at the break and cruising before substitute Tomi Juric completed the 4-0 rout.

The emphatic result, following Friday's 4-1 win over Kuwait, leaves no doubt about Australia's ability to win a first Asian Cup as they size up the knock-out stages.

South Korea's 1-0 win over Kuwait earlier means they are also six points clear in Group A. The Taeguk Warriors face Australia on Saturday to decide who finishes top.

Australia changed three players from last week's win in Melbourne but any concerns about the outcome in Sydney vanished in a first-half blitz.

The Socceroos took time to settle at an expectant Stadium Australia and Raed Saleh nearly gave Oman a shock lead when his powerful drive was tipped away by goalkeeper Mathew Ryan.

But Australia were soon finding space down the left and after Tim Cahill's header into the net was chalked off for a debatable foul, the opener looked only a matter of time.

The breakthrough came from a corner, when slack Omani defending allowed Trent Sainsbury at the far post to knock down for McKay, who gratefully swept in from close range.

Just three minutes later, on the half-hour, Kruse controlled Massimo Luongo's ball on his knee and steered his shot under the body of the diving Ali Al Habsi.

Australia had the ball in the net for a fifth time when Milligan buried Kruse's cross, but Japanese referee Ryuji Sato instead pointed to the spot for a foul on Cahill.

And Milligan put his frustration at the decision aside to coolly convert the spot-kick and put Australia 3-0 ahead and cruising at half-time.

Cahill came off for Juric after the break and the Western Sydney Wanderers striker slid in on Mathew Leckie's cross for Australia's fourth on 70 minutes.

The game was long over as a contest but it remained entertaining and Juric's goal-bound header was brilliantly saved by Al Habsi, and substitute Tommy Oar also came close.

Australia will now face South Korea in Brisbane on Saturday to decide who finishes top of Group A, a position which should earn a more kindly quarter-final.