The results will provide the government with an idea of what the public's views are. It is basically a survey.
There could be no biblical reason why JWs should not express there opinion, rather it would be a good opportunity for them to express it. For this reason, I don't think the org. Wants to make an official policy. It would confuse the JWs no end if the org said they could lodge their form.
It's not compulsory.
It's likely that the majority of Australians will vote yes but it will be interesting to see because they chose to gather opinions through a postal vote and not online, like they did for the recent statistics. Younger Australians rarely use the post, whilst older ones are comfortable with it. It's the younger Australians that are more likely to want same sex marriage.
In Australia, we have to post our letters at a post office or in a red box found occasionally on the side of a road. It's not all that convenient.