How difficult is it to legally transfer $800,000 (or any large amount over $100,000) from Pakistan to another country such as Australia, and what is the process?

You need to go to a Money Transfer Company (any licensed A money transfer company) and ask them to remit your funds. This is where the legal element would come into play as you would have to show proof on the source of funds. This would be a requirement for the receiving bank in Australia as well as the MTO (Money Transfer Operator) who would be transiting the transaction.

Your bank can assist you in finding an MTO to work with.