Johnston also supported his former club Liverpool's interest in Socceroos goalkeeper Mat Ryan and labelled talismanic striker Tim Cahill as the greatest Socceroo of all time.

The Socceroos are already assured of a spot in the quarter-finals before Saturday night's final game in the group stage against South Korea at Brisbane Stadium.

Ange Postecoglou's side has been ultra-impressive in its first two games, coming from a goal down to beat Kuwait 4-1 before completely outplaying Oman 4-0 last Tuesday to book its place in the last eight.

Even though the Socceroos were initially rated behind Japan, South Korea and Iran, Johnston believes they can ride their momentum through to the decider.

"I might not have thought that to start with but they've got such a head of steam up and such team unity, I think they can go all the way," he said.

"I thought they'd do well in the competition, but I didn't think they'd win it.

"There's a real belief and they're first to the ball all the time. I

Johnston will be in Canberra on Saturday night as the guest speaker at a soccer function hosted by the Belconnen United Football Club at the Woden Tradies.

He said the Socceroos were united and Postecoglou had brought them together.

"Football is all about lovely touch and sometimes you don't start to see lovely touch until the players are playing for each other and they find an extra half second here and there," Johnston said.

"Because people are thinking for each other and running off the ball, it gives the man on the ball two or three options.

"When players reach this kind of magic performance barrier where they are running their hearts out and things start to come off."

"Skippy" was a fan favourite at Liverpool from 1981-88, winning five league championships, scoring a double in the Reds' FA Cup triumph in 1986 and winning European and League Cups.

Johnston still has a soft spot for the Reds and hopes they add Ryan to their ranks alongside fellow Australian Brad Jones.

"They've had Brad for long enough, so they know the temperament of the Aussies," he said.

"I think the more Aussies that Liverpool buy the better for Liverpool.

"Let's see how he goes for the rest of the tournament but if he keeps performing like this, he could go anywhere."

While youngsters such as Ryan and Massimo Luongo are making a name for themselves at the Asian Cup, Cahill continues to lead from the front.

"Who would you rely in in the trenches in Gallipoli, I'm telling you it'd be Timmy Cahill," Johnston said.

"Timmy has seen them through three World Cups and pulled them out of the mire in three World Cups and that's just my objective assessment.

"He is the best player [ever] in terms of heart, guts and end result, there's only one, it's Tim Cahill."