I used to work for WW - shipping line that imports all the vehicles. Back when i was there, cars are already precleared with customs well prior to their arrival and are usually taken off the wharf within 24-48 hours so as to make room for the next batch - ie: by say another shipping line carrying Toyotas/Nissans etc (Bigg Japanese manufacturers have their own ships a lot of the time). Most Terminals in Aust have limited capacity for car carriers therefore once one unloads, the stock needs to be cleared asap as there is another one waiting to unload it's cars.
If you have ever seen the cars come off the ship, they look nothing like when u pick them up from the dealer. You would see that they are usually covered in a plastic film, a lot of the time, they are running on steel rims, not alloys and are covered in a fine white dust which is used to protect against salt air. Hence the detailing is where all the magic happens!
WW used to own it's own car detailing company - cars would be taken there to a massive undercover yard, and detailed as per dealer's instructions (ie: as per your order) and then delivered to dealer.
Things may have changed now in terms of detailing, but I am certain that the vehicles spend no more than 24-48 hours at the wharf.
Maybe a few variances to above now (ie - detailing more carried out by dealers.) but this is how it used to happen.