Turnbull Lied. Bill Rammed Through senate., page-105

That wasn't on the ballot though. It was specifically stated that there was no draft bill specifically tied to the plebiscite.

That was the Bill that the members were going to introduce as a private members bill but were persuaded to hold back in return for a public vote which had turned into a postal survey when Jana Wendt's son came up with the idea and it got support.

Members threatened to cross the floor as Liberals not in cabinet are allowed to do.
However those opposed threatened to blow up the joint if they did that. Hence the compromise was reached.