The way it works is they charge you the full interest payments on the anniversary date, and a few days later they credit back your account your offset component. A strange system, but it works fine.
So strange that I reckon you might have a fun time explaining it to the friendly auditor in some circumstances.
While those circumstances may or may not apply to you, its worth elaborating a little for those that see this as "just another fridge" coz its probably not.
A typical user of an IO facility with attached 100 % offset builds their Offset balance while not reducing their principal. They then convert the PPOR into an IP and use their offset balance to buy a new PPOR. This structure allows for holding the max tax benefit, while still giving u the benefit of reducing the interest bill.
The same structure is valid for an IP with an offset account.
Now Im assuming this little puppy doesnt work that way does it ?
Model
500 k IP
400 k loan, 200 k in the offset. The 200 k in the offset is TAX PAID money and not borrowed, and the borrower rightly uses their offset as a savings and trading account.
Assume rate is 5%, and lets work with generalised numbers and not specifics.
Monthly interest repayment is 1667.
Refund due to saved interest is 833.
The 833 refunded to the savings account may be regarded as a new loan, and the 833 may now no longer be deductible.
This process accelerates over time to eventually having converted all ur deductible debt to contaminated non dedutible debt. Sort of the opposite of a debt recycle strategy
Now, if one knows this is going to happen, there may be ways u can manipulate the product and its use to make it compliant, but I suspect there is no pre-sales disclosure of this issue, indeed id be surprised if there is disclosure at contract stage.
ta
rolf