Gay marriage advocates have vowed to keep campaigning as numbers mount against them in the first attempt at pioneering state-based same sex marriage.

The numbers for reform in Tasmania were lost tonight when the eighth voice against the bill was declared in the 15 member state upper house, the Legislative Council.

Australian Marriage Equality spokesman Rodney Croome said lower houser approval and some backing in the council were great achievements, given that Tasmania was last to decriminalise homosexuality.

"If opponents of this bill think they have put the debate to bed they are wrong," Mr Croome said.

Already struggling for numbers in the council, the Same Sex Marriage bill fell after key undeclared MPs raised constitutional doubts.

Hobart independent, Jim Wilkinson, said he believed same sex law would continue to be argued. "But I believe it will come back in the Federal arena. In my mind that's where it should be."

Among backers of the bill, Devonport MP Mike Gaffney said he was not concerned that the bill might be legally challenged.

"If the possibility of an invalid bill stopped us, very little reform would take place," Mr Gaffney said. "If we vote this bill down...the issue will not go away. Most importantly, the Australian community is alive to the issue of marriage equality."