If you are doing pre-cpl training flights at a flying school, do they have a operations manual that specifies how weight and balance calculations shall be performed? If they do, I would very much recommend complying with their procedure as the next ramp check could involve headaches for yourself and the flying school.
As far as private ops in your own aircraft, from a quick reading of the CAO sections posted by JT, my interpretation is CASA or a Weight Control Officer need to approve your system.
Loading data prepared in accordance with the provisions of this subsection may be approved by a weight control officer whose appointment covers the activity. Where a flight manual page is used as the load data sheet or to specify any required loading system, 2 copies of the same shall be submitted to CASA.
Note Such flight manual pages will be issued by CASA as a flight manual amendment.
The owner or operator shall keep in a safe place on the ground duplicate copies of all current approved loading data applicable to his aircraft.
As JT said, I think a WCO would find it difficult to sign off your excel spreadsheet ticks all the boxes, especially data corruption, however if you wanted to use it as a backup, or gross error checking tool in addition to the requirements I see no harm in that.