People found guilty of cyberbullying in Japan now face up to a year in prison under rules implemented Thursday, which were toughened up after the suicide of a reality TV star who had been trolled online.

Pink-haired professional wrestler Hana Kimura's death in 2020 prompted calls for stronger regulation of online abuse.

The 22-year-old was a fan favourite on Netflix hit "Terrace House", in which six young people share a home while looking for love.

But she faced a torrent of abuse online, reportedly including comments like "everyone will be happy if you're gone".

The revised legislation follows a passionate campaign by Kimura's mother, and now imposes fines of up to 300,000 yen ($2,200) or a year in prison -- increased from previous penalties of up to 10,000 yen in fines or 30 days' detention.

The punishments are contained in legislation on defamation, which defines the crime as "a display of contempt towards someone without demonstrating facts in a recognisable manner".

Justice Minister Yoshihisa Furukawa said the beefed-up punishments are intended to make clear that cyberbullying is a criminal offence.

"It's important that we work to eradicate spiteful insults that can sometimes push people to their death," he told a press conference this week.

Though the issue of cyberbullying had been raised in Japan before Kimura's suicide, the wrestler's death prompted domestic and international scrutiny and put pressure on lawmakers to take action.

Kimura's mother Kyoko has welcomed the new rules, telling reporters when the revision passed parliament last month: "I have a strong feeling of 'finally'".

But some free speech campaigners and legal experts are opposed to the change and have urged the government to ensure the tougher law is not used to target political criticism.

The Japan Federation of Bar Associations has warned that the legislation does not include any explicit provision protecting political speech.

"The prison sentence is inappropriate as it will stifle legitimate argument and threatens freedom of expression," the group said in a statement earlier this year.

Kyoko Kimura has acknowledged the concerns about infringements on free speech.

"I'm also strongly against misuse of the strengthened punishment in this sense," she said last month.

At least two men who sent hateful messages to Kimura have been fined, including one ordered to pay $1.29 million yen in May 2021 ($9,500 at today's rates) in a civil lawsuit over a message sent to Kimura's account after her death.

Earlier that year, a different man was fined a nominal amount over online abuse directed at the wrestler without the case going to trial.

"Terrace House" was cancelled after Kimura's death, but her mother has said those behind the programme "bear the heaviest responsibility" and that she is planning legal action against them.

Reports at the time of Kimura's death suggested producers had stoked conflict among members of the reality show that painted the wrestler in a critical light. (AFP)