Figures released to Radio New Zealand News show the bill for chartering a European ferry to fill in on the Cook Strait has topped $7.2 million.
The Swedish-owned Stena Alegra was brought in to replace the Aratere which lost a propeller last November and has been in dry-dock in Singapore for repairs.
It has been carrying passengers and freight most of this year apart from when it experienced its own problems including a gash in its side while docking.
The Stena Alegra will be returned at the end of June when the Aratere comes back into service.
The Aratere is on its way back from the Singapore shipyard where its broken propeller was repaired along with a problem with its rudder which was only discovered once it was in dry dock.
KiwiRail has yet to release the full cost of the repairs but has revealed the rudder damage cost $850,000 to repair.
KiwiRail said the Aratere's return to New Zealand was dependent on the weather but was expected to be sailing the Cook Strait again by 7 July. Its first week of sailing will be for freight only.