My understanding was that it continues to accrue at CPI.
If you cease to put in any tax returns in Australia and it is obvious you will never return I am sure at some point they will write off the debt. If you have any 'taxable australian asset' producing income (ie a rental property) then once that income exceeds the HECS repayment threshold you will repay debt, resident or not.
So don't worry, the HECS debt will be waiting for your return...but at least it only goes up at CPI.
But don't quote me;-)