Getting to Australia by boat can take several hours, several days or even weeks, depending on the type of boat, its speed and the point of origin. On a cruise ship, the most realistic time frame is at least a week.
The shortest distance between Australia and anywhere else is Papua New Guinea via the Torres Strait, and theoretically, this route could be traveled in only a few hours, though there are no scheduled services between the two countries. Two scheduled cruises offered as of 2015 are a 40-day cruise from Southampton via various European and African ports, and an 18-day cruise from Hawaii via New Zealand and French Polynesia. A cruise to New Caledonia offered by P&O can make the journey in as little as three or four days.