PAYG Payment Summaries, Interest , Dividends, Trust income... all of those automatically go to the ATO computers. If you lodge your tax return using E-Tax then you can download this information (except it doesn't guarantee all income, you still need to disclose any that isn't included). Accountants also have access to this information too from the ATO from a report they download from the Tax Agent Portal. I have clients who ring up querying income I have put in their return that they never told me about which the ATO has advised me of. Common one is interest being held in trust for kids.
Yup had one the other day. Checked his info on the Portal while he popped out to get his bank details and low and behold $6000 of undeclared Centrelink income.
I just tell clients that the ATO knows EVERYTHING so don't bother trying to pull a swifty.