Both of the navy’s $1.2 billion supply ships will be out of action until early next year as engineers struggle to identify the cause of defects that have crippled the new vessels for months.

The inability to use the faulty Spanish-built ships has frustrated naval chiefs and left the navy needing New Zealand and US tankers to refuel Australian destroyers and frigates on the high seas, and as China’s military pushes deeper into the Indo-Pacific.