A ban on gay conversion therapy that critics say could lead to Christian parents being jailed came into force in the Australian state of Victoria on Thursday.

The new laws make it a criminal offence to try to change or suppress a person's sexual orientation or gender identity. 

Anyone found guilty of the offence faces up to 10 years in prison.

The Change or Suppression (Conversion) Practices Prohibition Bill has come into effect a year after it was passed by the Victorian parliament. 

Victorian Attorney-General Jaclyn Symes said the laws would criminalise practices that are "total quackery".

"These laws send the clear message that these disgraceful practices are not tolerated in Victoria," Ms Symes said.

"We'll always have the LGBTIQ+ community's back - now the law does too." 

The Australian Christian Lobby (ACL) fears that Christians, including parents, risk being criminalised by the new laws.

It has warned that the wording in the reforms are so broad that parents who discourage their children from transitioning will be regarded as suppressing their child's gender identity. 

The ACL said the new laws also put counsellors and medical professionals at risk if they do not affirm a person's transgender identity, or their wish to take puberty blockers.

The ACL has distributed flyers to over 180,000 homes across the state of Victoria warning parents that they may go to jail under the new law.

"A new law has just made sure that if your child tells you they are transgender, you are prohibited from encouraging them to wait until they are older before pursuing potentially irreversible gender transitioning treatments," the flyer reads. 

"The Government can now receive, investigate and prosecute complaints from any person. Anyone could dob you in for having the 'wrong' conversations with your own kids."