Backpay happens when previous data has been overridden with new data, for example, positive and negative.
The way you identify the cause of backpay is that within the backpay tab, I recommend filtering first on the date column, and you would filter from oldest to newest date, which will give you the dates that the change was made for that is causing the backpay.
In this case, it's for the period from ____ to ____
Once you have the period the backpay is for, you will filter the pay items column to only include all payment pay items to identify the issue.
Pay items such as: Ordinary time, Overtime, Public Holidays, etc
Once you have Filtered on pay items, you will navigate back to the date column and filter again from oldest to newest.
In this case, the oldest date is ____, so this will be the first line
Once you have done this, you will see a common change that occurs, it negates and enters a positive back in. This means something was overridden for a prior period, the date identified in step 1.
You then check and identify the difference between the negative and the positive. You will check the costing, Award sources, Award types, Pay policies, etc
In this case, the difference is that it's negating from ____ and applying a positive to ____
This investigation tells you that the employee has had a change for a prior period.
Note: Backpay only ever occurs when something has been updated and changed incorrectly it is rarely a system issue so, please ensure that you do all of the filtering that is advised above and when you get backpay next time and you need assistance please ensure you list all of the above and advise what you have noticed.