Complex sentence: Under the "Australian Postal Corporation Act" 1989, letters up to 250 grams are reserved to Australia Post--other people and businesses can only carry them if they charge four times the basic postage rate.
Split into simpler sentences: Under the "Australian Postal Corporation Act" 1989, Australia Post has a monopoly of letters up to 250 grams. Other businesses can only carry them if they charge four times the basic postage rate.