The fare will give you a guide. And, for Domestic bookings it's generally D class. I believe I is used for connecting flights on some international itineries.
Do a 'dummy booking' select a date well in advance. Fridays are good as there are alway popular flights that almost never have any discounted Business Class availability. I class (or D Class) will be the lessor of two values that may appear.
E.g. Sydney to Melbourne - Fri 23 Feb 07 shows two prices for J; $433 and $592.
The $433 is showing where it is available. The $592 appears when only full J is available.