An Australian man deported from Thailand after supporting anti-government Red Shirt protesters says he was beaten in prison and feared the Thai government would plant drugs on him.

Conor Purcell arrived in Sydney this morning after more than three months in prison.

He said he intended to return to Thailand but was critical of the support he had received from the Australian government while in prison.

"I'm going to go through the FOI Act to find out what exactly was happening."

He said he was beaten by seven other prisoners in an "ordered assault" and denied medical attention for almost two weeks afterwards.

After the attack he could not walk and had blood in his urine, he said.

He said he was kept in a cell with almost 40 prisoners, some convicted murderers awaiting appeal.

"On the second day, they had seven criminals beat the living shit out of me," he said.

"It was the most cowardly assault you could imagine. They went crazy on my spine, my back, my ribs, my kidneys.

"They [the Thai government] could plant whatever they wanted on me."

Mr Purcell - who accepted political charges against him but did not plead guilty - said he did not know a lot about Thai politics before he became involved with the Red Shirt but felt compelled to help after seeing the violence at protests in April.

"I had to speak out against what I'd seen ... what I saw on April 10 was absolutely horrendous. Compared to the Bloody Sunday massacre [in Northern Ireland] this was way worse, this was incredible," he said.

"I am almost certain they will prevent me from returning. I will do whatever I can ... and then try to go back to Thailand."

Mr Purcell was met by a group of about 15 supporters from Thai Red Australia, carrying banners that called him a hero and welcomed him home.

"Welcome home Conor Purcell," read one poster. And, on the reverse, referring to Thai Prime Minister Abhisit Vejjajiva: "Abhisit is a tyrant. We don't want murderer PM. We don't want mafia ruler government in Thailand."