Lawmakers in New Zealand are expected on Wednesday to give final approval to a bill legalizing gay marriage.

Big crowds are expected to be on hand to witness Labour MP Louisa Wall's marriage equality member's bill receive its third reading in Parliament.

The measure received overwhelming approval at its committee stages last month. Wednesday's final vote is considered a formality. Bills are rarely rejected at this stage.

Wall, who is openly gay, has previously said U.S. President Barack Obama's endorsement of marriage equality gave momentum to the bill.

“Marriage belongs to society as a whole, and that requires the involvement of the whole of society,” Wall told colleagues before last month's crucial vote. “The role of the state in marriage is to issue a license to two people who love each other and want to commit to one another formally. That's what this bill does.”

Family First, a conservative group opposed to the proposed legislation, has threatened to oust any MP who votes for the bill.

“The definition of marriage should stay as traditionally and commonly conceived – not one manipulated by politics and political correctness,” said Bob McCoskrie, a spokesman for Family First.

The measure won't go into effect until after it receives Royal Assent from the Governor General, Lt. Gen. Sir Jerry Mateparae, Queen Elizabeth II's representative in New Zealand.

If the bill is approved, it is expected to take effect in August.

New Zealand currently recognizes gay and lesbian couples with civil unions.