THREE out of four Australians believe voluntary euthanasia should be legal, a poll has found.

But despite levels of endorsement that have hovered between 75 and 85 per cent for many years, pro-euthanasia reformers admit to having been consistently outgunned by their opponents, resulting in five failed attempts to pass legislation in two years.

The latest poll by The Australia Institute found 75 per cent of people agreed that if someone with a terminal illness who is experiencing unrelievable suffering asks to die, a doctor should be allowed to assist them to die. Thirteen per cent did not agree and 12 per cent were not sure.

As well, the poll revealed 65 per cent of people who identified as Christians were in agreement as were 71 per cent of Coalition voters, 79 per cent of Labor voters, and 90 per cent of Green voters.

Josh Fear, the deputy director of The Australia Institute, said the high support among Christians was striking, as was the 73 per cent support from people aged 65 and over. ''If anyone in the community would have thought about it and had strong views, it would be people feeling their mortality,'' he said. ''Support in the oldest age group was slightly less than average but still very much a majority.''