Harry Kewell was spared official Socceroos training as a disciplinary storm whirls around him which could end his World Cup.

Kewell is the subject of an official FIFA investigation into his verbal tirade against a referee after Australia's 2-0 loss to Brazil.

He faces being banned for Australia's crucial World Cup match against Croatia on Thursday (FRI AEST) - and possibly longer - after German referee Markus Merk complained about Kewell's behaviour in a post-match report.

He has been given until 5pm AEST today to submit his case to FIFA's disciplinary committee, which will then rule on what action to take against the Liverpool star.

Kewell was a no-show at a public training session at the team's southern German base of Oehringen, held in front of more than 4,000 people who turned the stadium - and the entire town - into a sea of green and gold for the day.

Assistant coach Graham Arnold explained away Kewell's absence as to receive treatment on his ongoing groin complaint.

"Harry's had the groin problems and we felt there was no use him coming down the field and doing nothing," Arnold said.

"He's back at the hotel getting round-the-clock treatment and doing a pool session - nothing at all to do with the (disciplinary) situation." FIFA has promised a decision on Kewell's punishment will be made prior to the Croatia match in Stuttgart.

Arnold said Australian officials were combing DVD footage of the incident to see if there was anything they could use to help Kewell's case.

But it doesn't look good for him, with photographs and television replays clearly showing him striding purposefully towards Merk, mouthing off and pointing his finger at him - the sort of anti-referee histrionics FIFA frowns heavily upon.

It is believed suspending Kewell for at least the Socceroos' next match - vital to Australia's chances of qualifying for the second stage of the tournament - is the most likely option if he is found guilty as charged.

Australia can secure a spot in the World Cup round of 16 with a draw against Croatia in Stuttgart, provided Japan does not produce a miracle win by at least two goals over Brazil in its last game against Croatia on Thursday (FRI AEST).

Defender Tony Popovic is also set to miss the rest of the World Cup after breaking down with a calf strain during the Brazil game.

The carnival atmosphere at the training session was marred by a bizarre two-person nude protest - with a man and a woman carrying a sign saying "I'd rather go naked than wear Australian wool".

A female pitch invader also ran the gauntlet of security guards to seek autographs midway through the session, before being forcibly escorted out of the stadium.

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