The procedures for once-off payments are identical to those for commencements. The only difference is that the payment is one lump sum payment and no fortnightly basic commenced. This will be decided by deterimantions issued.
The tax will also need to be calculated and entered as a fixed tax entry. This is to ensure the correct tax is deducted (refer 3.4.5). DoF can not spread the tax over the period determined in the back payment if they have not issued any payments for this period. The tax will need to be entered, including the current pay periods tax in the Fixed Tax field for an urgent payment in page 2 of AIND.CM. It can also be entered in AEMP.CM as an ongoing fixed tax amount, but this MUST be deleted after cut off.
The commencement and cessation date will need to be the first and the last day of the pay period. The ATO are not concerned when the payments were earnt, only when they were received.