Just four out of 15 Legislative Councillors have indicated they will vote for the same-sex marriage bill when it is debated on Wednesday. Six are publicly against. 

The bill passed the House of Assembly with the support of Labor and the Greens late last month amid scenes of celebration, but the government's task to get it through the Legislative Council is getting harder.

Federal Parliament yesterday rejected a bill to legalise gay marriage.

In a blow to the Tasmanian government, Nelson MLC Jim Wilkinson, who had been expected to support the bill, said yesterday he had developed serious concerns in the past 48 hours.

Mr Wilkinson said that he had asked himself whether he would want his child to be able to marry if they were gay.

``Of course I would. But would I want second best for my child? No and that's what this is.''

He said the legislation failed to achieve equality because it created a totally different version of marriage so as not to contradict the federal Marriage Act.

Critics of state moves for gay marriage have argued that federal legislation would override any state laws.

``It's like your vinyl coat compared to a leather jacket. From a distance they look the same but when you get closer you realise it's not the same, it's a knock-off. They want equality, they're not getting equality with this,'' Mr Wilkinson said.

If Mr Wilkinson joins those voting against it, the government will need the support of all the remaining four undeclared MLCs to get it over the line. 

Even those leaning in favour have raised doubts. 

Rumney MLC Tony Mulder is expected to vote for the legislation saying he doesn't care who marries who, but yesterday said he would first need questions answered.

``There are a few issues that need to be resolved particularly the constitutionality and whether it actually achieves marriage equality.'' 

Huon MLC Paul Harriss, who plans to vote against the bill, predicted it would fall over.

``The best the government can hope for is a tie, seven all,'' he said. 

In the event of a tie at the final reading, convention dictates Legislative Council president Sue Smith should use her casting vote to uphold the status quo. 

Tasmanian Gay and Lesbian Rights Group spokesman Rodney Croome said he had been speaking to many of the independent MLCs and urged them to keep an open mind. 

Mr Croome said he did not know how those sitting on the fence would vote. 

``It's still too early to tell,'' he said.

Many MLCs would prefer the issue be dealt with at the Commonwealth level, but yesterday the federal government rejected a bill to legalise gay marriage 42 to 98. 

Three of Tasmania's five MHRs voted in favour of the legislation. 

The Senate is today expected to reject a similar bill tabled by Labor Senator Carol Brown.