BHP says it erred by building shafts for its first potash mine that were bigger than required, but told investors the consolation would be much cheaper expansions of its Canadian mine in future decades.

BHP has spent close to $US3 billion ($4.1 billion) over the past eight years sinking shafts at the Jansen project in Saskatchewan, and announced in August that it would spend a further $US5.7 billion over the next six years completing the first phase of the mine.