Complex sentence: Vast tracts of land were leased until required for agriculture, once the land was surveyed it was put up for sale and the "Squatters" as they were known had to buy their runs or move on.
Split into simpler sentences: Vast tracts of land were leased until required for agriculture. Once the land was surveyed it was put up for sale and those already living there ('squatting') had to buy their runs or move on.