It's easy to check your last bill and compare, that should give you a definite answer to your change of bill amount.
Compare bill reading type, actual or estimated.
Compare usage, easy to do where the amount of various power KwH are shown.
(If an estimated bill on an old analogue meter, check current reading to see if that was massively inflated . . . common.)
(Noted though you were on monthly, so possibly have a digital meter which should not require reads.)
Compare tariffs. The most likely culprit if readings are ok. Power has gone up so much with big hits, and also minor creeping rate increases in the past year and a half or so.
You don't say what normal monthly bills used to be, but your account (if it keeps similar usage) is going to be a bit over $500, so it's trending down, possibly a seasonal change, or you are aware of the last bill and consciously trying to cut power use where you can.
It's best not to just swap providers, because you might be going from a bad to worse situation.
Google > energy made easy < the govco site to assist you in finding the best plan for your location and available energy retailers, talk to AGL on phone or chat and ask them about plans.
They (and probably all retailers) don't make it easy to get on the best plan, although their bills used to have a little note saying 'you seem to be on the best plan for your usage', now it says to go and check the energy made easy site.
Good luck with the search for an energy cost answer.
