Best underfloor insulation?

What insulation would be best to go under the floor for a pole house being built in the Adelaide Hills? (it can get mighty cold overnight in Winter, so it needs to be good).

If the house is being built use SilverFlor this is doubled sided foil product that comes in 600 wide rolls and you staple in-between the joists.
If you are going to cover in the area at a later date then when the ceiling goes up install highdensity sound batts.

Having grown up and lived in many suspended timber floor houses - best option is to do as oziwi says, insulation and line it - it makes a lot of difference.

This link might help you along with your decision.

http://www.timber.org.au/resources/R-values%20floor%20elements%20Part%204%20of%204%20-%20Ed%201.1.pdf

Steve