tax question

Rightio, so say i buy several parcels of particular stock at various prices, then i sell a parcel of the same stock, for CGT, can i select which initial parcel purchases i sold. eg.

Buy

20000 MPO at 10cents
20000 MPO at 15cents
20000 MPO at 20cents

Then i sell 20000 MPO at 30cents

can i match the 30cents sell with the 20cent buy, even though the 20cents buy was the more recent?

All trades done within 12months.

So i guess my question is, do i have to use the "first bought, first sold" method, or can i mix and match to give the lowest CG's.

cheers.