Safety warning systems at the entrance to Auckland's Manukau Harbour will not be upgraded, despite a container ship running aground on the notorious Manukau Bar.
The coastal container ship Spirit of Resolution was damaged when it grounded on the bar in big seas five years ago.
A Transport Accident Investigation Commission report published today said the ship crossed the bar in deteriorating sea conditions that were already at the margins of safe operating limits.
The commission considered whether better information on predicting and measuring wave heights over the bar would have made any difference.
But it decided that unless the volume of shipping increased, current systems were sufficient to maintain maritime safety for Manukau Harbour.
The report said bar crossings were notoriously hazardous for vessels of all sizes.
The sea conditions were forecast to exceed the 4 metre maximum permissible wave height over the bar, and stay that way for at least another two days.
The ship's master decided to head towards the harbour entrance, knowing that he had the option of anchoring and waiting for better conditions, the report said.
Information relayed to him indicated wave height of up to 4 metres and increasing, but the bar was still open and the ship began to cross.
The Spirit of Resolution was almost clear when the wave height suddenly increased and the ship foundered before it was carried sideways by the current out of the main channel and into shallower water.
Most of the rudder sheared off when it struck the bottom and the ship's bow was damaged. The waves also damaged a number of containers on deck.
The ship was manoeuvred into deeper water, where it lay for about two days before it was towed to New Plymouth and then on to Lyttelton, where it was repaired.
Retired sea captain Gilbert Inkster, who has crossed the bar many times, said it was sometimes difficult to judge the conditions but it should not be attempted in bad weather.
He said crossing the bar was made worse because it was so far off the shore, it was shallow and affected by extreme currents running across it.
"It's not a very nice place to use in bad weather and if there's any doubt you stay inside at anchor or stand off the bar until it improves."
Mr Inkster said while he had crossed in bad conditions, a wave height of 4 metres did sound extreme.
Since the accident the Spirit of Resolution has been returned to its owner and the Pacifica Transport Group has stopped operating into Manukau Harbour.