The Cook Strait ferry Aratere is to get a $40 million refit to make the boat 30 metres longer and increase passenger capacity by about 300 people.

The refit is part of KiwiRail's Turnaround Plan which includes boosting Interislander's ability to carry rail freight as well as passengers and commercial vehicles.

The boat will leave for Singapore on April 13 where it will be cut in half and a new 30-metre body will be installed as well as a new bow. Vehicle capacity will be increased by 30 per cent and rail capacity by 27 per cent.

The five-month refit was expected to be finished in time for the start of the Rugby World Cup in September, Interislander general manager Thomas Davis said.

A boat has been leased to cover the Aratere while it is out of service to minimise disruption to passengers.