Yes, flights to Tasmania are perfectly legal in an RAAus aircraft and even by direct flights as a further option to the recommended island hopping, CAO 95.55 states: (www.casa.gov.au/download/orders/cao95/9555.pdf)
You probably wouldn't be in the middle of the Bass Strait at 8,000ft unless IFR or climbing/descending to a VFR Cruising level. If the engine fails, fly the plane, Aviate. Navigate. Communicate. Depending on the aircraft and its particular performance, you possibly could glide to land. Don't forget there are islands that you can follow. If it were too dangerous, do you really think CASA would allow these flights? Class E extends to FL180 over most of the Strait, so in a GA aircraft, assuming the aircraft can go to the FL's, and you have oxygen (and whatever else would be required), a higher altitude can be used. RAAus pilots can also request an exemption from CASA if they wish to as per Section 9 of CAO 95.55 (See above link).
The ERSA contains pretty much everything you need to know, ATC are there to help so don't be afraid to contact them. Don't forget your life jacket and other mandatory safety equipment. The crossing has been flown by plenty of pilots and the plane doesn't know you are over water, they biggest thing to watch out for is weather conditions - Don't be afraid to wait it out because conditions in the Strait can get very nasty, real quick.
Oh, and don't forget to take your passport and make sure you don't take anything that you shouldn't due to Quarantine... http://www.quarantinedomestic.gov.au/destination-tasmania.html