A bill to legalise euthanasia is not about disability or mental illness but giving choice to people who might otherwise be "dragged off for one last futile treatment", its sponsor said at the ChristChurch Transitional Cathedral on Thursday night.

ACT Party leader David Seymour went toe-to-toe with the Reverend Dr Graham O'Brien, who argued for better palliative care and mental health services.

Seymour told about 100 attendees that his mother had polio "so my whole life was defined by disability".

He said he was raised by someone who thought going to the beach was "nuts", and avoided the supermarket because she struggled with the 50 metre walk from the car park.

"I would never write a bill that somehow discriminated against someone with a disability," he said. "This is not about them, this is about people who are in a state of decline and have been for a long time."

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