I think I'd disagree Leper; the point of a retrospective award is to put you back in the position you should have been in the first place.
I'd push back; start with the company who underpaid you, they should have paid you in 2017, you should not be out of pocket even further and the award compensation should take that into account.
A tax agent to review your affairs will cost a lot less than the 13k you've lost in tax and would be money well spent in an overall review if nothing else.
Ultimately, you may or may not succeed; but it's worth some small time investment to appeal in the first place.