South Korea smashed eight past lowly Laos and Asian champions Australia battered Bangladesh 5-0 as the continent's elite made light work of the region's plucky outsiders in World Cup qualifying on Thursday.

Japan again looked disjointed in attack but still had far too much quality for an outclassed Cambodia in a 3-0 victory in Saitama, while former Asian champions Iraq kicked off their Group F campaign by downing Taiwan 5-1 in Tehran.

Only Hong Kong bucked the trend, holding big brothers China to a surprise goalless draw in a politically charged encounter in Shenzhen, played out in front of a heavy police presence, to stay top and unbeaten after three games in Group C.

Hong Kong are part of a group of weaker nations that have taken advantage of the revamp of Asian qualifying that has thrown them in with the continent's classiest following a plea for more matches.

They along with Guam, Palestine and Singapore had taken surprise group leads after two rounds across the eight pools but Thursday's early results showed most are unlikely to be there when the second round concludes in March.

New Tottenham Hotspur signing Son Heung-min struck a hat-trick as perennial qualifiers South Korea claimed the biggest win of the night, 8-0 over Laos.

"After just 15 minutes, the match was pretty much in our hands," Korean coach Uli Stielike was quoted as saying by Yonhap News.

"But our players competed hard until the end, and I think we're becoming a better team with each passing day."

Australia, who beat the Koreans to win the Asian Cup in January, also had a straightforward night as they eased to a 5-0 win over Bangladesh in Perth despite missing a number of first team players from the starting lineup.

The Socceroos were 4-0 up in 29 minutes against a side who barely had a touch of the ball, while home goalkeeper Adam Federici was virtually redundant.

Japan enjoyed similar dominance but profligacy in front of goal meant they could only record a 3-0 win over Cambodia for their first victory in Group E.

Shinja Kagawa recovered after a terrible miss from three meters out with the goal gaping to round off the scoring in the 60th minute of a game played almost entirely in the Laos half.

"I think the players felt a little bit of pressure," coach Vahid Halihodzic was quoted as saying by Kyodo News.

"We could have scored more today with all the chances we had; we cannot be happy with just three goals."

Udinese fullback Ali Adnan added to his burgeoning reputation with a brilliant cut back and blasted finish as Iraq, buoyed by goalkeeper Noor Sabri coming out of retirement, proved too strong for Taiwan.

Guam and East Timor will hope to avoid the plight of the other minnows when they face daunting away trips to Iran and Saudi Arabia in two of 10 matches being played later on Thursday.