SINGAPORE (AP) _ A seven-story luxury hotel set sail Monday on a 2,175-mile voyage to open for business as a floating resort off the coast of Australia.

The 12,000-ton structure departed aboard the Dutch heavy-lift ship Mighty Servant 2 om hotel, owned by Barrier Reef Holdings Ltd., is anchored securely inside the Great Barrier Reef, a massive coral formation stretching 1,250 miles along Australia's northeastern coast.

Its destination is the central lagoon of the John Brewer Reef, 45 miles from the port of Townsville. Well-heeled vacationers will need 90 minutes to reach the Four Seasons Barrier Reef Resort by catamaran from shore or 20 minutes by helicopter to the rooftop helipad.

The five-star hotel will concentrate on short-staying guests, most of them American, with room prices ranging from $240 to $580 a day, its promoters said.

Amenities on attached floating piers will include tennis courts, a swimming pool, a gymnasium and sauna, a disco, restaurants, bars and an underwater observatory for viewing marine life. A submarine is available for coral excursions.

The hotel was designed by the Swedish-owned Consafe Engineering (Far East) and built in the rig building and repair yard of Bethlehem Singapore (Pte.) Ltd., which is 70 percent owned by Bethlehem Steel of the United States and 30 percent by Developmnent Bank of Singapore.

The yard since has received inquiries concerning similar structures from the United States, Australia, New Zealand, the Caribbean, Latin America and Pacific islands, said Bethlehem General Manager Milton H. Leubecker.

Based on technology developed for building offshore living quarters at oil rigs, it cost $21.5 million and took 16 months to build. Made mostly of prefabricated steel modules, the complex is largely self-sufficient.

It has two sewage treatment plants and a desalination system able to produce up to 152 tons of fresh water a day, with power from three 1,200- kilowatt generators.

''It is likinds of up to 100 mph, Leubecker said.

Originally scheduled to have been delivered in October and to have opened by now, the floating hotel was held up in a dispute over the cost of extra work. Executives said it was handed over by the builders earlier this month after an out-of-court settlement was reached.