CAO 95.55 covers this for recreational aircraft. You will find that at the end of the ops manual.
The link is http://www.comlaw.gov.au/Details/F2011L00617
The relevant paragraphs are below.
7.1© subject to paragraph 7.2, the aeroplane must not be flown over a body of water at a horizontal distance from a suitable landing area of more than:
(i) the distance (not greater than 25 nautical miles) that the aeroplane can glide in case of engine failure; or
(ii) 25 nautical miles — if each occupant is wearing a life jacket and the aircraft carries a serviceable radiocommunication system and the equipment referred to in subparagraph 3.2 (a) or (b);
3.2 (a) an approved ELT, or an approved portable ELT, as defined in regulation 252A of CAR 1988; or
(b) a personal locator beacon that has been approved by CASA for use with such an aeroplane.
7.2 In spite of the limit of 25 nautical miles mentioned in subparagraph 7.1 ©, an aeroplane to which that limit would otherwise apply may be flown between Tasmania and mainland Australia, in either direction, by a longer route if taking advantage of safer weather conditions.