You can only legally collect with the permission of the land owner or manager. That means working out which department manages which its of public land - local shire, roads department, forest company/manager, DAF or other state gov dept. Many bureaucrats find it far easier to deny any applications that to work through any possible issues so permits can be hard to obtain. There is a very real fear of backlash if someone causes an accident or some other damage or gets injured while on their land.
Much easier to get permission from private landowners.
Much depends on what you want to collect. Most native plants are protected and can't be taken without a permit. Some are even protected on private land. Feral, weedy species are easier as they have no environmental protections.
Our local club has made connections with our district landcare groups. We offer to help by removing invasive species from areas the landcare groups are working to clean up and revegetate.
Pine forest managers won't be happy with someone digging up their trees and are usually worried about possible injuries or damage. Pines tend to be tall and straight so not worth collecting. Some self seeded trees growing on rocky or steep banks may be worth looking at.
Your other option is to collect without permission and hope no-one finds you. Wear hi-vis workwear and look like you are supposed to be there.
Wherever you collect, make sure you don't do any damage or cause problems, fill in all holes, etc. and remove rubbish so you, and the rest of us, don't get a bad name.
I find that private gardens are a far better source of trees for bonsai and you may find your energy better directed there than to the bush. They tend to be easier to dig, have a much higher rate of survival and often have better bonsai characteristics. Suitable species include: azalea (any time of year) Callistemon, melaleuca and banksias (warmer months), privet, plums, cotoneaster, pyracantha and many more. Tell family, neighbours, friends and work colleagues that you are a crazy bonsai person and would love to help when they need trees/ plants removed when someone is cleaning up a garden or building, etc. many will just think you are mad but sooner or later you will hit the jackpot.
Could also be worth making contact with local garden maintenance people and arborists. You may need to remind them a few times as they won't understand what you are after at first.
I think most finds are just random luck - being in the right place at the right time. Driving past council workers digging out callistemons to make way for a drain. Also much easier because they will have a machine to do the digging for you
I've found that the vast majority of trees I've looked at are not worth taking home. Bonsai really need taper and some trunk movement which very few wild plants have. I usually look at many hundreds before digging 2 or 3 of the best. A few weeks later I usually wonder why I ever bothered to bring most of those home.
