An Australian journalist being tried for defaming the Royal Thai Navy says he is fearful, but hoping he will be found not guilty when the verdict is handed down.

A senior Thai judge at the Phuket criminal court will deliver his ruling on Tuesday in the case against Alan Morison, 67, formerly from Melbourne, and Thai reporter Chutima Sidasathian.

Morison and Chutima face jail terms of up to seven years on the defamation charges as well as breaches of Thailand's tough Computer Crimes Act if found guilty.

"We remain extremely concerned about the outcome, but hopeful that we'll be found not guilty," Morison told AAP.

"Anything less than a no guilty verdict would be bad for freedom of the media really, internationally."

The Royal Thai Navy brought the charges against the reporters when in June 2013 online newspaper Phuketwan republished excerpts from a Reuters report alleging Thai security forces were involved in the smuggling of Rohingya refugees from Myanmar.

The Reuters reporters later were awarded the prestigious Pulitzer Prize for international media reporting. Thai reporter, Chutima, has assisted foreign media, including the Reuters reporters, in arranging key interviews on stories covering the Rohingya issue.

Morison says the case has wider implications for the media.

"The case revolves around a paragraph we reproduced word for word from an international newsagency (Reuters). Whatever decision is made has the potential to affect everybody who reproduces news from any other source," he said.

In a court submission Morison claimed errors in translation from the English version to Thai. The English version refers to 'Thai naval forces' but was translated as the Royal Thai Navy.

The Thai and international media have regularly raised the issue of human smuggling in Southern Thailand and Malaysia in an illegal trade worth millions of dollars.

Thai authorities earlier this year uncovered mass graves of up to 200 smuggled Rohingya that led to a major crackdown by Thai authorities and the arrest of more than 100 people including Thai officials.

Morison and Chutima have received widespread support from international human rights and media groups, including the New York based Human Rights Watch, Paris-based Reporters' Without Borders, and Australia's Media, Entertainment and Arts Alliance.

Senior Australian officials including foreign minister Julie Bishop and Ambassador Paul Robilliard have also raised the case with Thai officials including the Prime Minister Prayut Chan-ocha.

But legal sources in Phuket told AAP the legal outcome was far from clear. The Thai military-backed government may wish to make an example of the journalists. "I don't know what's going to happen," they said.

"The key in this case is the current government, that's the key for me."

A senior deputy chief justice is due to hand down his ruling on Tuesday, September 1.