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Thanks for your post and welcome to the Community.
By the sounds of it a new account has been established in your name and this is the current process whenever someone is wanting to take financial responsibility for either gas or electricity at an address with AGL.
What should happen from here, the first reading should take place and this will trigger a final invoice for your housemate's account and then you will receive any future ongoing invoices in your name.
If you would like please send me a private message with your account details and I can confirm that this is the case. Please use this private message link here.