Interislander sailings on the Aratere ferry have been cancelled due to urgent repairs.

It means there will be no Interislander passenger ships sailing for the rest of the day, which could affect any anti-government protesters making their way from the South Island to the capital.

KiwiRail executive general manager Walter Rushbrook said there was an issue with the ventilation system for the Aratere's engines.

"We are working to make the repairs as quickly as possible, but the system is difficult to access.

"At this stage we expect Aratere to resume sailing tomorrow, providing the work proceeds as planned."

Rushbrook apologised for the disruption to passengers and freight customers.

The Cook Strait ferry service had already been reduced to just one passenger ship as it struggles with an increasingly unreliable and ageing fleet.

KiwiRail was forced to reduce the service between the North and South Islands during August and September.

The Aratere has only just returned from wet dock work and the Kaitaki is undergoing dry dock work until September 28.

The Kaiarahi ferry was "catastrophically" damaged and is in need of European specialist assistance when its gearbox failed last year. It has been out of action ever since.

While another ship, the Valentine, is sailing at the moment, it's for freight only.

So, there are no Interislander passenger ferries running for the rest of the day.

Bluebridge ferries are still running.

Police were aware of individuals from the South Island who wanted to join the protest planned in Wellington tomorrow, a spokesperson said.

"We have been in communication with organisers to discuss potential routes and plans.

"Police recognise the lawful right to protest. We also recognise the rights of the public to go about their lawful business."

Police were monitoring the situation accordingly and staying in contact with Marlborough District Council, the spokesperson said.