There are plenty of old walnuts trees growing around Adelaide. Some kind souls probably planted them for the generations to come. I don’t think they’d come up on their own. The one growing next door gets visited regularly by a flock of white cockatoos who have a great time munching away on them. There are also a few trees I know about on public land that can be foraged. I went to one of those trees to see if they were ready for harvest and managed to collect a full bucket in about 15 minutes.
It’s better to collect them while the whole husk is green as the rotting husk damages the inner nut and shell. Before you start thinking about dehusking the nut make sure you keep in mind that the husk will create a pretty permanent dark brown stain on any surface it comes in contact with, including your hands so use gloves. To dehusk, get the shell and roll it under your foot on a hard surface. The husk will crack and sort of lift of the shell. If you add too much pressure the nut will crack. Once you’ve rolled it around a bit pick it up and peel off the husk. Most of it should come off. Then place it in a bucket of hot water. Soak the nuts in the hot water for a couple of hours. This will soften any left over husk and the ones that sink are the good ones. Scrape of the rest of the husk with steel wool or a knife and place in a tray to dry.
Let the nuts cure for a few weeks in a shaded ventalated space outside. Protect them from rats and birds but ensure they get good ventilation. The lighter ones in the above photo were the one from the green husks and the darker to black ones had more rotting husk on it they are most likely no good for eating but the light ones should be good.