A vote on a bill to allow voluntary euthanasia in New South Wales is expected to go down to the wire in the state's Upper House today.

All MPs have been given a conscience vote on the legislation, which would allow terminally ill patients over the age of 25 to end their own lives with the help of doctors.

The bill's backers were buoyed by the momentum created by similar laws being sponsored by the Victorian Government, but the ABC understands the NSW Upper House vote is extremely tight.

It is likely to be decided by a small number of MPs who are still wavering, including Deputy Nationals leader Niall Blair.

Premier Gladys Berejiklian and Opposition Leader Luke Foley have both indicated they will vote against the bill if it reaches the Lower House next week, where it looks likely to face an even more difficult passage.

Nationals MP Trevor Khan, who introduced the bill to Parliament, said terminally ill people should have the right to control how they die.

"We all accept that palliative care is very important and that it helps the majority of people, it just doesn't help everyone," he said.

"This is a bill that seeks to address the issues hopefully of a relatively small percentage of people."

Mr Khan said many of the bill backers supported voluntary euthanasia because of their own life experiences.

"They've watched their relatives and loved ones die undignified and pretty horrible deaths in many cases," he said.

'Cannot legislate on how to end a life'

Labor's health spokesman Walt Secord said he would be among those voting against the bill.

"I believe that parliamentarians cannot codify legislation on how to end a life," Mr Secord said.

"It's not possible to put in sufficient safeguards and protections to prevent abuses from the laws."

On Wednesday a group of palliative care doctors and nurses opposed to the bill came to Parliament to make a last-ditch plea to undecided MPs to vote against the bill.

Under the scheme, proposed patients must be over 25 years old and expected to die within 12 months.

They must be assessed by a psychologist or psychiatrist and have their decision signed off on by two medical practitioners, including a specialist.

Close relatives can challenge patient eligibility in the Supreme Court.