Maritime New Zealand says it takes seriously an incident which has seen the Interislander ferry Arahura restricted to freight-only sailings until further notice.
The vessel has been taken out of passenger service after one of the cables attaching a lifeboat to the ship came away during a training drill in Picton on Monday.
Maritime New Zealand spokesperson Steve Rendle said lifeboat safety is paramount on a ship and if something goes wrong it can be serious. Such ships regularly carry about 250 passengers.
Mr Rendle said the international vessel classification society - Det Norske Veritas - has given the go-ahead for the Arahura to carry freight for now, but it could be until next week before it is safe to resume carrying passengers.
All of the cables that hold up the four lifeboats onboard the Arahura must now be replaced at a cost of about $20,000.
KiwiRail on Tuesday defended its safety testing programme. Chief executive Jim Quinn said they routinely test all of the ships' equipment and the lifeboat cables are put under extreme stress to test safety limits.
Mr Quinn said it is unlikely the wiring would give way during a normal rescue situation.
Interislander general manager Thomas Davis said the cost of installing the new cables would be close to $20,000 and an investigation is underway as to why the relatively new wiring broke.
Mr Davis said passengers are being rebooked on the Kaitaki or the Stena Alegra, which is on lease from Poland to replace its other broken down ferry the Aratere, which lost a propeller midway through a crossing from Picton to Wellington on 5 November last year.