Impossible to say
The practice note on Federal Court legal costs is here.
In Australia, the general rule is that costs follow the event (loser pays) and usually on a “party and party” basis rather than an “indemnity” basis.
An indemnity basis means that the loser pays the full costs that the winner had to pay to their lawyers, experts, etc. A party and party basis means the loser pays the scheduled rates which are typically one third to one half of the actual rates. The current rates are here. As an example, the senior council rate for prep work is up to $740 per hour; the actual fees charged by a QC will probably be in the order of $2,500 per hour.
The court will typically make a costs order (saying who pays and on what basis) at the time of the judgement and then the parties will agree the actual costs. If there is a dispute on this, they can ask the court to resolve it (potentially incurring more costs).
The actual fees for using the court are nominal - less than $1,000 per day and are insignificant in the scheme of things.
To hazard a guess, Mr D is likely to be up for $100-200k for his own legal team, $30-50k for the government’s team (with the government likely to have spent considerably more) and about $5k for the court itself.