Moody was not, truth be told, a terribly skilful prospector: he was hoping for nuggets rather than panning for dust. Too often the ore-bearing gravel slipped through the netting at the bottom of the cradle, only to be washed away; sometimes he emptied his cradle twice over without finding any flakes at all. He was making what the diggers called "pay dirt," meaning that the sum total of his weekly income was more or less equal to the sum total of his weekly expenditure, but it was a holding pattern he could not sustain. (p. 525)And again:
"This claim," he said, gesturing. "Pay dirt only. Very small gold." (p.531)As far as I can tell, "pay dirt" is an Americanism that emigrated to the Australian and NZ goldfields. Whether the connotation changed from vast riches to a sustainable return, or whether the usage in this book simply reflects Catton's understanding of the idiom is unclear to me, but it certainly seems to be a different usage of the term.
We made only pay dirt during our first year in Otago. (p. 709)