Say someone has a 100% offset account on their mortgage. This means that if they have:
Loan amount: $150k Offset balance: $100k
...then their interest is computed as if they only owe $50k on the loan. The offset account doesn't accrue interest on its own in this case (as in, the balance doesn't increase), but effectively it's as if the offset account earns interest at the same rate that's applied to the loan. Usually that rate is above what's available on a standard saving account, which is what makes the offset account an appealing option to begin with.
Anyhow, my question is around what would typically happen in the following scenario:
Loan amount: $150k Offset balance: $160k
The balance in the offset account now exceeds the amount payable on the loan. The account-holder pays $0 in interest on the loan, but is also left with a "surplus" of $10k over and above the balance due on the loan.
The question is, would interest typically accrue on this amount, and if so at what rate (as in, would it usually be the bank's standard cash savings rate, the rate of the home loan, or something else?)? Can it make financial sense to fund an offset account over and above the amount outstanding on the loan being offset, or is there basically no reason to ever do something like this?