We are the 'first' people.
OK? who did you massacre and enslave to make you 'the first'?
Was someone in the way?
A lot of new arrivals (came in from the west) moved south to easier living over the 32,000 or so years they were coming in, so the northern territory tended to be like it is today, relatively unpopulated. Chances are not much resistance was encountered as they moved in.