Ahhhh you're just down the road mate! How close to the coast are ya? The closer you are, the less chance of frost. If you're within 20-30 odd km, you'll probably never see frost. We can get cold, but it's rare. 10-12c overnight is about average and 22c is daytime average on the coastal fringe up here. That is practically summer in Melbourne/Tazzie
Obviously the further inland you go, the colder night temps you'll get.
I can't help ya with autos either sorry mate, I have a couple of freebies that will be my first attempt at them.
The sun will trek north in the sky so if you have anything north of your greenhouse, it may block some of the sun's path. Its intensity is much less, so the more uninterrupted hours of it the better. Still possible to get a suntan here in winter, so I'm backing that it should be strong enough to grow something.
Great thing about the weather here is that it's consistent. When you say average up here, it means basically that -2/+2 degrees every day. No Melbourne type of swings where you can go from 40c to 14c in a heartbeat. Sure we get cold snaps and variances, but in winter a cold snap means cold at night and majestic through the day (shorts/t-shirt in the midday sun). All those glorious post card pics you see of our area, are taken in the winter months. You can tell by the colour of the sky (crisp, dark blue) whereas the summer sky is a very light hazy type of blue. I believe this is all the moisture in summer and the cooler dry air I mentioned before in the winter.
If it rains this time of year (wet season), it can rain for weeks so your greenhouse will come in very handy all year round. Storm season has finished/ending now, not sure how long you've been up here but the storm season brings on some great smokin' times on your verandah watching nature's spectacular (and at times scary) light and sound shows. Your greenhouse is also going to be very handy during these months