NSLO Conversion Instructions, page-13

‘How do I identify the particular shares or units I have disposed of’.
If you have the relevant records (for example, share certificates), you may be able to identify which particular shares or units you have disposed of. In other cases, the Commissioner will accept your selection of the identity of shares disposed of.
Alternatively, you may wish to use a 'first in, first out' basis where you treat the first shares or units you bought as being the first you disposed of.

From here:
https://www.ato.gov.au/General/Capi...entifying-when-shares-or-units-were-acquired/