From Wednesday, terminally ill Victorian adults who meet 68 criteria will be able to ask their doctor for a lethal combination of medication.
A letter signed by four Victorian bishops warns it will mark "a new, and deeply troubling chapter of health care".
"We cannot cooperate with the facilitation of suicide, even when it seems motivated by empathy or kindness," the letter signed by the Melbourne, Ballarat, Sale and Sandhurst bishops said.
Under the voluntary assisted dying laws, patients must be of sound mind and have less than a year to live, or under six months for those with neurodegenerative conditions.
Their suffering must also be deemed "intolerable", and they must make three, clear separate requests to die and be assessed by two experienced doctors.
Medical professionals are able to conscientiously object.
Pro-Life Victoria plans to protest the assisted dying laws on the steps of parliament on Tuesday night, saying they will devalue palliative care and "legitimise suicide" for vulnerable people.
