Complex sentence: If, however, the drift continued northward the ship could, once freed from the ice, proceed to New Zealand for repairs and resupply before returning to Cape Evans in September or October to complete the second depot-laying season.
Split into simpler sentences: If, however, the drift continued northward the plan would be for the ship, once freed from the ice, to proceed to New Zealand. After repairs and resupply it would then return to Cape Evans in September or October for the second depot-laying season.