The men admitted to breaching COVID-19 restrictions when they came before Adelaide Magistrates Court on Wednesday.

It was alleged Jacob Todd and Nicholas Batty, both 29, Alexander Moore, 22 and Sam Gledhill, 26, boarded the train in Melbourne on Monday night.

They were arrested on Tuesday after it arrived at the Adelaide Freight Terminal and spent a night in custody.

All four were placed on 12-month good behaviour bonds but had no convictions recorded.

Under current rules, people from Victoria, or anyone who has travelled through Victoria in the past 14 days, are not allowed into SA unless they have an essential traveller exemption.

The four men are now expected to be held in SA to complete 14 days of supervised quarantine.

Premier Steven Marshall described the incident as something out of a movie.

"It's an incredible situation. It's like a movie storyline from 20 or 30 years ago," he told ABC radio.

"Kids stowing away on a train and getting across the border."

Mr Marshall said he did not have the details to hand on what checks were made on freight trains.

But he said SA police had done an incredibly good job of patrolling the state's borders.

Health Minister Stephen Wade said given how the four men made it into South Australia, he would speak with the attorney-general about any changes to laws that might be required.

Under the current situation, jail terms are not an option for anyone breaching COVID-19 regulations with the maximum penalty a $20,000 fine.

"These people have come in through the back door," Mr Wade said.

"We need to make sure our laws are flexible enough to deal with these intrusions."

Australian Associated Press