First issue is, do you have to cash out your shares? Or can you exchange them for shares in the parent company? If shares, you may want to defer cashing everything, you can slowly sell shares as you need them, keeping yourself in a lower tax bracket (capital gains taxed as normal income in Aus). Or you can borrow against them.

You need a professional financial and tax advisor. Always go independent (not affiliated with a large bank/institution) and fee-based (not commission).