You cannot use the main residence exemption for an empty block of land (unless it is only temporary such as in the construction of a new PPOR).
If you subdivide part of a block of your PPOR for the purpose of resale, that part is no longer exempt - but holding title to multiple blocks to use as your PPOR might be OK (< 2ha).
Generally the ATO deems the cost base to be allocated completely to the land where you acquire premises intending to demolish the building - i.e. no capital loss for demolishing.
Demolishing and subdividing might constitute a business of development, in which case ordinary income will apply - not CGT. This depends on how extensive your involvement is.
Do get some advice appropriate to your individual circumstances BEFORE acting.
Cheers,
Rob