Firstly not a good idea to supply address details here.
You need to contact AGL directly.
Here are some things you should know:
You should contact AGL to discuss your issues.
I presume you mean the account is billed up to 3 April.
If you have once every three month account (90 days) then your invoice is reasonable.
You still have to pay for a supply charge ($1.00 a day), if you have electric hot water then that is still being heated.
You still were using power for (90-17) 73 days in that billing cycle.
For the 90 days usage that works out to be $3.41 a day.
Get your last bill and compare that.
( Just find the bill total and divide it by 90 to get your daily usage).
Hope this helps, but still suggest that you contact AGL, if doing the simple sums find that you have excess usage.