An Australian man freed after spending over a month detained in Tokyo is calling for an overhaul of the country's family custody system.

Scott McIntyre, a sports journalist based in Tokyo and former SBS reporter, was arrested on charges of trespassing when he went to his estranged in-laws' apartment building to look for his children, whom he said he'd not seen for months.

He was released on bail last week after pleading guilty to the charges, and on Wednesday was given a suspended six-month prison sentence.

Unlike most developed countries, Japan has no joint-custody system after divorce.

Reuters

"The state of Japan has failed me and failed other parents in providing any information or any access to our children," McIntyre said at a press conference on Thursday.

"When we try out of frustration to ascertain this information, we are the ones arrested, detained and placed in appalling conditions and tortured.

"I was held in facilities with all sorts of people who had done some serious kinds of crimes."

McIntyre claimed that when he complained about conditions, including 24-hour light, he was threatened with solitary confinement or a straitjacket.

He said in Japan, "children are being used as pawns in this situation" and it was "human rights abuse" towards the children.

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McIntyre, who reported on football at SBS, was arrested on 28 November for entering the common area of the building where his wife's parents live in late October in a bid to find his children, now aged 11 and 7.

Japan's judicial system has drawn global attention with the lengthy detention - and subsequent fleeing - of former auto executive Carlos Ghosn in what critics have characterised as a "hostage justice" system.

Earlier, McIntyre emerged from the courthouse wearing a shirt with the words "Stop Parental Child Abduction" printed in both Japanese and English.

He told reporters his children had been taken from him when he split with his wife in May last year.

"I haven't seen my children now for almost 250 days.

"All we want is that... Japan joins the rest of the civilised world in implementing a system of joint custody."

He said he had made numerous requests to the police and his wife's lawyers - the two are going through a divorce mediation - to let him know whether the children are safe, but that those were ignored.

The day of the illegal entry, he had been worried about his children in the aftermath of a typhoon that ripped through the region, he said.

Prosecutors said McIntyre's wife had claimed physical violence by McIntyre towards their daughter, which he denied, and material presented by the prosecution was dismissed as irrelevant to the trespassing charge.

It was also not clear why he was arrested more than a month after the illegal entry, or why he had been detained for so long.

An earlier request for bail was denied on grounds that he could destroy evidence or flee the country.

Additional reporting: Reuters