The full cost of getting the Interislander ferry Aratere back in service is expected to be between $20 million and $30 million.
The ferry has been out of action since losing a propeller midway through a crossing from Picton to Wellington on 5 November last year.
On Saturday it left Wellington for Singapore where it will undergo dry dock repair work, expected to take about four weeks.
KiwiRail chief executive Jim Quinn said the overall financial impact will include the loss of passenger, rail freight and commercial vehicle revenue as well as the hire of a replacement ship from Poland.
"It's not just a cost impact, it's a loss of earnings impact as well," he said.
Mr Quinn said the Aratere is due back in service in May.