A court in Thailand today sentenced an Australian man to three years in prison for insulting the country's revered monarchy, in one of the few convictions of foreigners under harsh lese-majesty laws.

Harry Nicolaides' sentence had been halved from six years by the five-judge panel in Bangkok because he apologised and pleaded guilty to the charge, which carries a maximum jail term of 15 years.

The 41-year-old looked tense as he stood in orange prison-issue overalls – shackled at the ankles and handcuffed – while the judgment that he had insulted King Bhumibol Adulyadej, 81, was read out.

Nicolaides, a teacher from Melbourne, who has lived in Thailand for several years, was accused of insulting the king, the crown prince and the monarchy in a self-published 2005 novel, Verisimilitude.

The offending passage – read out in court – told of scandalous gossip about a fictional prince and the judge said it "suggested abuse of royal power". The novel has sold just seven copies in Thailand.

Earlier a tearful Nicolaides said he had "unqualified respect for the king" and had never intended to insult the man who is the world's longest-serving monarch.

"I would like to apologise," he said, as guards led him from the court. "This can't be real. This is an Alice in Wonderland experience. I really believe that I'm going to wake up and it will all be gone."

His case comes amid a rising tide of lese-majesty accusations and prosecutions, apparently spurred by Thailand's political polarisation as complaints can be filed by anyone against another.

Nicolaides still does not know who filed the complaint against him. He was arrested in August as he tried to leave Bangkok's international airport, oblivious to an arrest warrant issued in March over the book he describes as a commentary on social and political life in Thailand.

He was indicted in November and denied bail. He said he endured "unspeakable suffering" during the five months he was in custody awaiting trial.

As he left the court, the shaven-headed Nicolaides gave no indication if he would appeal against the sentence or apply for a royal pardon, saying only he wished his "family the best".

Two years ago a Swiss man sentenced to 10 years for defacing images of the king with spray paint while drunk was given a royal pardon and deported after serving a month in prison.