After experiencing the joys of trying to get a tyre repaired on a Saturday afternoon in western Queensland (Hughenden), I'd replace them before leaving home. Most tyre failures occur in the last 30% of their tread life, so better to have newish tyres at the start of the trip.
That said, it was the factory tyres with less than 3000km on them on my mates ute that had to get repaired. The crappy 3/4 worn out chinese things I had 3/4 of the tread worn off I couldn't kill with a stick..... come to think of it, go with the old ones. The only time I have holed 2 tyres in a weekend, they were brand new tyres
But my mates tyre was a Bridgestone and the two I destroyed were also Bridgestones, so come to your own conclusions there.
If the road turns to slop, the extra tread on new tyres comes in handy.