Make sure you get light from the sun immediately when you wake up and when there’s no sun, get a lightbox or any other kind of bright light therapy.
There are technically medications for it but your best bet is to get lots of light early, limit bright light at night and use things that make the screens orange/amber colored and ensure that all light in the house is nothing past warm yellow (no green/blue/purple) like 6hrs before bed and try to use dimmer lamps rather than harsh overhead lights.
I’m pretty sure SAD is more of a circadian rhthym issue rather than the big D necessarily. It means you need to get your cycle back in order and once you do, you will likely feel much better.
Also be sure to get your Vitamin D3 and K2 as well. They are all related and healthful in isolation but very useful as a combined plan of attack.
It is pretty much unheard of in Australia, and I had no idea it was a thing until I went to Tassie for a couple of years for one of my degrees. I see how it could happen, down there, but it is too sunny here in NSW year round for it to have a kick-in time. Perhaps you need to move to the Lucky Country?
There’s definately places in NSW where you could experience SAD. Places in the Central West for example; Orange and Bathurst get quite gloomy during the winter.