This post comes from an unexpected discovery when my husband and I voted at a recent election. It was a local election and neither of us was able to vote on the actual day so we attended a Pre-Polling booth. There was a HUGE line but the Election Officers spotted the disability sticker on the car and Owen in the back. It turned out this booth offered "Assisted Voting". Hubby was allowed to go straight in and vote and they sent a portable booth out to the car for me to put on my lap and vote (yes in the car) so that I did not have to unload Owen. Within 10 minutes we were gone. Impressive and helpful!.
This lead me to investigate further and there is a postal voting category called a "General Postal Voter" and you can tick a box that says you need to do a postal vote, each election, as you are "caring for a seriously ill or infirm person". We ticked that box, uploaded the forms and explained in the info box that the infirm person was our disabled child who had fluctuating health. Our application was approved. So for federal elections we will get a ballot paper in the post and for state and local elections a form to tick to confirm we wish to do a postal vote, then a Ballot paper in the post. More information can be found at: General Poster Voter
So the crux of this is apply to be a General Poster Voter or take advantage of Assisted Voting if offered at your Polling booth (apparently they list it on the where to vote list they prepare for each election).