First of all, thanks for joining the Community!
'Debit adjustments' and 'Credit adjustments' are applied for a variety of reasons, though the most common reason is an overcharge or undercharge on your previous bill that has been identified after the fact, possibly due to an estimated reading. If you'd like us to jump into your account and provide you with a more precise answer, you're welcome to get in touch with our web chat team
I had something like this come up too. No explanation. Just hidden away on the second page. Even help desk at two levels couldn't tell why it was there. All my relevant readings were actual, not estimated. I am losing confidence in the AGL billing systems.
It comes across as a sneaky grab for money. Will be looking elsewhere soon I think.