When an employer processes the payment summaries for the year they also send a report to the ATO with all the details. So the ATO has access to your figures from that employer. Years ago I had an employer refuse to resend a copy, even though they have to, my accountant was still able to do my return, but could not give me a figure for a refund. I would suggest if you are not doing so already go to an accountant to get it all sorted, especially as there are past returns to complete.