Australia's best chance of legalising gay marriage -- in Tasmania -- is in serious trouble, as nervous politicians baulk at setting a precedent on the hot topic.

A bill to legalise same-s-x marriage has passed the state's lower house and goes to the upper house tomorrow, with a vote expected in the richly decorated 19th-century chamber late Wednesday night or on Thursday. A Crikey analysis last month found the bill's chances were line-ball. Crikey revisited the issue this week and found the prospects for the bill have dimmed as two MLCs have changed their minds.