Must be minimum 700mm deep, all cables in conduit (I ran phone and power in the same trench). Sparky must lay the cables and must be there when backfilling to lay the plastic danger tape over at the 700mm level.
We dug about 800mm deep, put the cables/conduits on the bottom, then back filled 100mm and put the danger tape on that. Not that I plan I digging in that area, but at least I know there's a little safety factor between the warning and the shock.
If you can get an operator on site for a good price (my backhoe was $75 an hour for man and machine) your big expense is the sparky to pull the cables in the conduit.
I don't know if there are specific regs for the length of run you can do underground, but my understanding is as long as it's shown accurately on your plans your sweet.
Energex only care about their wires from their poles to your first pole. That's why I did the board on the pole first and get their BS out of the way.
The board on the pole is the same as if it was the board in your house, you just need a qualified sparky, no need to deal with the power supplier.