I am not a lawyer or an immigration expert however I have experience with the AU IMMI system, both directly and indirectly. There will be some who disagree with my take and that's ok - I won't argue with them because my opinion is not perfect.

It's very important that you don't get caught in a lie when you deal with IMMI. If you do, it's over, and you may never get a chance to even visit Australia in the future. Even if you lie and get a visa, if it is eventually uncovered everything could be cancelled. So in general, it is best to be truthful.

That said, admitting a criminal record will hurt your chances of getting your visa (although it is definitely possible you can get it with those offences declared).

My general advice would be to query every public record you can. See what is available on your record. If it is available via public record, you are best to declare it. Because it's likely they will find it. If you can't find a record of it, you can decide whether you want to declare it or not - it's likely if you don't, they won't find out. But nothing is guaranteed.

I wish you the best of luck!