Conversion therapy can cause serious harm, says Justice Minister Kris Faafoi, who is introducing the government's planned law to ban the practice.

The government today introduced its Conversion Practices Prohibition Legislation Bill, which gives full details of planned legal changes including the penalties people would face for breaching the law.

Conversion therapy involves attempts to change a person's sexual orientation, despite that not being possible.

Speaking today, Faafoi says he was really happy to introduce the legislation.

"The practices are widely discredited by science," he says.

He says conversion practices can cause serious harm including severe depression and anxiety, difficulties forming relationships, substance abuse and risk of suicide.

Faafoi says survivors of conversion practices have been reported to show symptoms of post-traumatic stress disorder.

"There can be no justification for trying to force someone to change their sexuality or gender identity."

The Salvation Army and Anglican diocese have denounced the practices, he says.

"This bill will not outlaw a person's right to hold personal beliefs about sexual orientation or gender identity ... and it will not stop parents or religious leaders from offering support."

He says the criminal offences in the bill are intended to prevent particularly serious harms from the practice.

"No one should be able to change someone's expression of who they are."

Complaints can be made to the Human Rights Commission for performing conversion practices on another person under the civil law.

The law is set to bring New Zealand into line with other countries including Germany and several Australian states.

The bill will have its first reading in early August and the government aims to have the legislation passed by early next year.

Faafoi says he did not think it had taken long to get the bill introduced, having made the commitment at the election last year.

"What it does do is prevent harm from continuing to happen ... we know that conversion practices don't work, we know they cause harm."

He says he did not know what help was available to help those who have been harmed by the practice, but the government offered a range of services. He says the government does not want cases to end up in court, the idea is to prevent conversion therapy practices.

Faafoi says the National Party had supported the legislation in principle, he had spoken to the Māori Party, and the Greens had expressed support for the bill also.

"We want to make sure that this piece of legislation gets passed."

He says the bill allowed people to express their religious views, but people should not be allowed to change who other people are.

• RNZ