In the final they will face the winners either Adelaide United or Western Sydney Wanderers, who will play the second semi later in October.

The Sky Blues have been turned into a ruthless side by Arnold and they showed no sympathy to their Victorian NPL opponents.

Within four minutes the A-League champions, fresh off a 1-0 win over Melbourne Victory last Saturday night, had their noses in front.

The part-timers of South Melbourne had not played a competitive match for a month or so following their quarter-final triumph and the end of the NPL Victoria season, and their ring rustiness showed.

The hosts failed to clear a forward pass, the rebound was played forward and Alex Brosque found former Socceroo David Carney who drove past Nikola Roganovic with ease.

South tried to hit back but they lacked fluency, not surprising given their layoff. In contrast, Sydney were fit and confident after their successful start to the season.

Marcus Schroen headed just wide from a cross by South wide man Nicholas Epifano, but that was as good as it got for the former Melbourne City attacker. Within five minutes Schroen was limping to the bench, unable to continue, after suffering an injury.

South coach Chris Taylor, who only returned from holiday on the eve of the game, was forced to reshuffle his pack early, but his team's task became even harder a few minutes later when Sydney doubled its advantage.

Dutch defender Jordy Buijs was allowed far too much time and space when he embarked a bullocking round from defence, charging forward as the South midfielders backed off. His pass found Brazilian striker Bobo, who picked his spot to fire past Roganovic.

Sydney was comfortably in control at this point although South's Matthew Millar found Milos Lujic in space, but the forward's shot lacked any power.

Sydney skipper Alex Brosque's header was saved by Roganovic before the keeper then dealt with a shot from Carney and another from Milos Ninkovic.

South gave themselves hope shortly after the restart when they pulled back a goal, Leigh Minopoulos showing good technique to control Kristian Konstantinidis flighted cross before driving past Andrew Redmayne in the Sydney goal.

Millar then fired over after a good South break as the part timers sniffed a chance to get back on terms, and the same player caused Sydney palpitations when his long range effort almost caught Redmayne out after the keeper came well off his line.

South, energised by their goal, began to show plenty of self belief. The underdogs started to move the ball more quickly and take on their Sydney opponents.

Lujic tested Redmayne with a good shot on the turn but Sydney snuffed out hope of a comeback in the 65th minute when Bobo scored his second, scuffing the ball home after a Brosque cross.

Sydney made it four with 16 minutes remaining through Sebastian Ryall, and substitute Matt Simon emphasised the gulf in class with a header six minutes from time.