If it was just you as usual, it would be difficult to work out such a difference.
The person staying there for a week may have had more 'loose' power usage habits, but hard to imagine it could make that much difference.
A few things to check . . .
Was it an estimated read ?
Depending on meter type . . . if you have a digital meter, then this bill is likely correct for data taken via the meter.
If you have an analogue meter, those are usually a quarterly actual reading taken, and the 2 months between are estimated, or a consumer can upload a reading just before each months bill is due, so it is more accurate.
It could be that an estimated read was well overestimated, and if an analogue meter you should be able to read it yourself and upload a reading via the website login or app, if it is indeed now less than a reading shown on the bill.
If the meter reading is a little higher now than shown on the bill, then it's likely the reading on the bill was correct.
Have you dropped off any set plan you were on ?
Usually you'd get a letter to say your contract for 12 months is coming up, and you then go onto the standard plan, which might (likely) be dearer.
Has your plan or have tariffs changed ?
A little investigation on your part now could prepare yourself for contact by AGL, either here by private message, or via phone or app chat service.
Good luck getting to the bottom of this.
