Yeah, it's complicated. Basically:you have these standard...

Yeah, it's complicated.
Basically:
you have these standard guidelines that average investor would be familiar with and then whole lot of circumstances that ATO might determine as tax avoidance or cheating.
And a lot of them aren't very clear or logical or well known and spelled out on ATO publications.

I presume you sold 20,000 shares at higher price then bought them back for lower.
So ATO can audit you and determine that you're cheating CGT by 'washing shares' or something.
Best to do what accountant says or ask for second opinion from another tax agent.
Penalties can be quite costly if you get audited