On an Offset Loan account with Redraw the interest is calculated as the Principal Balance - Redraw Balance. If the Redraw Balance for a given period is >0, then the interest calculated is going to be lower than the expected interest for the period. The difference between expected and actual interest should be available to be Redrawn from the redraw balance.
At the moment, Mambu's Redraw Balance functions like an Offset Deposit Account. Mortgage lenders make the interest saved due to the redraw balance available for redraw.