The state-owned rail company is also considering investing millions of dollars more into passenger carriages.
KiwiRail leases two of its three Interislander ferries the Kaitaki and the Kaiarahi, and it owns the Aratere. The latter has been dogged with problems, just last month a report found the Aratere's propeller - which fell off in 2013 - had not been fitted properly.
Now, the company is considering whether it should buy or lease a ferry when the Kaitaki's lease is up in 2020.
The Kaitaki carries up to 1600 passengers, and is the largest of KiwiRail's fleet but also the largest ferry operating in the country.
KiwiRail chief executive Peter Reidy said it was a matter of making sure the company had the right configuration and the financing of it could be worked out a little later on, "so whether you're buying it or leasing it you're still contributing to capital."
He said the Interislander proved its worth as a critical piece of the country's infrastructure after the Kaikōura earthquake in November. "The Interislander, it's proven once again that it's a critical lifeline for New Zealand, that Cook Strait is a very important infrastructure for New Zealand, so we'd be looking to invest to make sure we've got the capacity for New Zealand whether we lease or buy, that's a decision for our board or our government in future years."
KiwiRail is also considering buying an additional nine or 10 passenger carriages for its rail service which could cost between $15 and $20 million and it's looking at investing particularly in the TransAlpine Christchurch to Greymouth route.
Mr Reidy said the company's strategy was to connect journeys across its network on rail and by sea and its passenger numbers are increasing dramatically.
He said KiwiRail carried about 1 million passengers a year and that number is growing 15 to 18 percent per annum with a lot of international visitors, particularly from China, starting to use rail as a way of seeing New Zealand.