I've only ever paid my bills quarterly and in full, but I do know AGL offer bills quarterly or monthly, may you can just switch to monthly billing ?
Otherwise, if a payment plan was available from AGL and is now withdrawn, the only other option I have seen mentioned on their bills is to call and arrange a plan, which would be tiresome to have to do every quarter.
Maybe they have withdrawn how you did it due to monthly billing offered ?
That would be same as you paying it off each month, or is it that they want bill smoothing or something else applied ??