Australia confiscated guns a couple of years ago so how did he happen to have one? Though not guilty of murder, could the authorities still charge him with possession of a firearm?
Because they didn’t confiscate ALL guns.
Semi-autos and pistols, but bolt-action long rifles can sill be owned, but registered. I believe some revolvers also.
Not if it's a legal firearm and he has a licence, and it's registered.
Australia does have some strict gun control laws but it's fairly easy to own non semi-automatic long arms. Owning semi-automatics or hand guns is a little harder. Owning automatics or high calibre/high capacity hand guns is possible, but difficult unless you have a particular work related reason to do so.
The extent of Australian gun control has often been exaggerated overseas into some idea that all guns have been banned. In fact, there are millions of firearms still legally held in private hands and somewhere over a million people with gun licences (about 10% of the adult population).