Migrant worker representatives and activists in Hong Kong have continued to express anger over the alleged torture of an Indonesian maid by her employer, amid the launch of a criminal investigation of the case by the police.

Indonesia's Migration and Labour Ministry on Wednesday condemned as "unacceptable" the abuse of 23-year-old Erwiana Sulistyaningsih.

It said she will return to Hong Kong to testify against her employer, the South China Morning Post newspaper reported on Wednesday.

Sulistyaningsih flew back to Indonesia on January 10 to seek treatment after her alleged abuse and torture for eight months.

Local migrant worker groups said she remained in hospital in central Java, with her condition improving.

"We are very angry with the situation, it's not our first case," Eni Lestari, chairperson at the International Migrants Alliance, said.

"This is just too much. We should question why Indonesians helpers are not even able to help themselves when they are abused."

Eni said her group planned to stage a protest in the coming days.

Other groups called on the governments of Hong Kong and Indonesia to launch an investigation after local media reports said officers were not treating it as an active criminal case.

"This is very big," said Mia Sumiati, chairwoman of Komunitas Migrant Indonesia which runs a shelter home for abused maids in Hong Kong.

"Those responsible should go to court. We also request our government to help her come back to Hong Kong so she can report to the police."

Late on Tuesday a police spokesman said a criminal investigation had been launched.

Police had previously refused to investigate the case, the South China Morning Post had reported earlier.

Hong Kong is home to nearly 300,000 maids from mainly Southeast Asian countries - predominantly Indonesia and the Philippines - and has come under growing criticism from concern groups over their treatment.

In November, Amnesty International, the UK-based human-rights organisation, condemned the "slavery-like" conditions faced by thousands of Indonesian women who work in the Asian financial hub as domestic staff and accused authorities of "inexcusable" inaction.

It found that Indonesians were exploited by recruitment and placement agencies who seize their documents and charge them excessive fees, with false promises of high salaries and good working conditions.