I am 72 and still work 22.5 hours per week, as I was doing when I applied for and was granted a partial age pension. Because I was born in the USA Centrelink forced me to apply for a US Social Security payment, which was granted, and my Australian age pension reduced accordingly. Although my wife was born in Australia, she did work for 18 months in the USA and was also granted a small SSA payment. Centrelink somehow didn't calculate this, nor a work bonus payment that I received and reported to them, so we ended up with a combined "overpayment" of A$3000 going back over 5 years. We finally agreed to a 'repayment' plan that saw us paid about A$30 per fornight less than we would normally get. I absolutely refused to pay a lump sum amount to them as I knew that it would never be recovered if it was discovered that they were in error.
We received calls from two different offices, one in Hobart and one in Tamworth, neither of whom communicated with each other. The supposed overpayments went back at least five years. It was agreed by both offices that I had done all the right things by uploading documents and payslips, and using a tax agent to prepare my returns, but the 'debt' stood as I had neither the time nor the resources to fight it. The rules and processes are so complex that it is impossible for a 'client' to adhere to them, while at the same time it appears that the system is easily rorted by those in the know. It was a demeaning experience to try to defend yourself when we had done everything 'by the numbers'.