An Australian man under investigation for allegedly assaulting an American tourist has said he is stranded in legal limbo in Spain and has pleaded for government help.

Adam Dalton, 24, said he spent six weeks sharing a jail cell with a double murderer and five drug dealers after an incident during a night out with friends in Barcelona.

Mr Dalton, of Westmeadows in Melbourne's north-west, said he was released from a Barcelona jail earlier this month after paying almost $20,000 in bail.

He has not yet been charged, but has been required to surrender his passport to a Spanish magistrate, Australia's Department of Foreign Affairs and Trade (DFAT) said.

"He is under investigation for assault ... he appeared before a magistrate and he's under instructions not to leave Spain pending a court appearance at a date yet to be determined," a DFAT spokeswoman said.

The construction worker began a European holiday in July with seven teammates from the North Heidelberg football club, travelling with the company Busabout.

The group arrived in Barcelona on August 26 and headed out on a pub crawl.

After leaving a bar, Mr Dalton said he was talking with an Australian woman when Mark Waschman, a Texan whom he had never met, crashed into the left side of his back.

"My instant reaction was to push him off," he said.

"There was no malicious action involved."

Mr Dalton went to another nightclub and said he did not look back to see that Mr Waschman had cracked his head on the footpath.

Five days later, police found Mr Dalton at his hostel and took him into the station, where he spent the night alone in a cell, he said.

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