Sun Cable: Cannon-Brookes wins battle for world’s biggest solar and battery project, page-79

It would be somewhere around 15-20%.
it would have an LV/MV transformer onsite, MV/HV transformer to enter the transmission lines, an HV/UHV transformer to export at 1000+KV. and the same on the other end. So 5 transformers at ~.8% each gives 4% losses. The MV retic for such a large farm would have~3%, HV to Darwin another 3%. That's 10%. The undersea cable would have 5-10% losses depending on operational methods and willingness to spend on Capex.
This doesn't include the use of batteries or any parasitic loads from system strength equipment.
As others have pointed out, that's a huge voltage drop which creates real problems. They'll probably need intermediate reactive compensation stations which would add to Capex and losses.