At least three Labor senators will vote against the legalisation of same-sex marriage, but have backed down over religious exemptions to allow the bill a smooth passage through the Parliament.

South Australia's Don Farrell, Tasmania's Helen Polley and Queensland's Chris Ketter – all linked to the conservative shop assistants' union – have signalled their intention to vote against marriage equality in coming days.

Jacinta Collins, another "shoppies" veteran from Victoria, will effectively abstain by pairing her "no" vote with "yes" supporter Gavin Marshall, who is on secondment to the United Nations. Deborah O'Neill, from NSW, is also expected to vote no, or to abstain.

The number of socially-conservative Labor senators is not sufficient to endanger passage of the bill, and their decision to vote against the bill is accepted by the party, which has granted all MPs a conscience vote on the issue of same-sex marriage.