I don't live in Australia to know what the practice is there, but here in the US it's not uncommon for a company to advertise one price, but then tack on a variety of fees, taxes, and surcharges to the final bill that actually pay for the service and cost associated with providing it.
"Oh your rate plan for your [cable|internet|cell phone|whatever] is 19.99. Your monthly bill is $82.45 after adding in sales tax, USF fee, phone number|IP rental surcharge, regulatory compliance fee, capital improvement fee, lobbying surcharge, fee just to fuck you because we can fee, CEO compensation package fee, legal defense fund for when we get in trouble for all these fees fee, fee to make more money fee, fee fi fo fum fee, and finally are you even listening any more fee."
By making the company include any of THEIR business taxes as part of the service fee, they can't just bury their additonal costs as a line item and pass it on to the customer without affecting the price of the package.