Big ships entering and leaving the notorious bar on Manukau Harbour on Auckland's west coast will be restricted to sailing one hour before high tide.

The restriction is likely to be posted in the next two days to avoid the potential catastrophe that could have struck a 4500-tonne freighter that hit the bar as it left the harbour on Saturday.

The Spirit of Enterprise lost its rudder when it was hit by a rogue 3m wave, which lifted the bow and caused the stern to hit the bar, the ship's owners, Pacifica Shipping, said.

The rogue wave theory surprised Auckland harbour master Captain James McPetrie who said the 3m swell running at the harbour entrance was not unusual.

He said he would "find it difficult to sustain" calling it a rogue wave.

"I was surprised to hear that," Cpt McPetrie said today.

Port authorities and the Maritime Safety Authority had discussed the restriction, which would apply as soon as it was posted, probably today or tomorrow, he said.

"We are about to issue an instruction to all masters of certain vessels that they are only to cross the bar from a time from one hour before high water to high water."

The restriction would apply to all ships with a draught (distance from the keel to the waterline) 6m or over.

Until now ships' masters have been able to make their own decisions on when they would enter and leave port.

The Spirit of Enterprise, now being towed to Lyttelton for repairs, used its bow thruster (propeller) to steer the ship out to sea to await help.

It left Onehunga at 12.45pm on Saturday and would have arrived at the bar about an hour after high tide.

Cpt McPetrie said if a single-screwed ship lost its rudder crossing the bar it would be in serious trouble.

Twin-screwed ships can be steered in a clumsy but reasonably effective manner by varying the revolutions on each engine.

"The whole thing had the potential to turn into a catastrophe. Had she lost her main engines then it would have been an entirely different game," he said.

The bar was the site of the worst shipwreck in New Zealand's history on February 7, 1863, when the British warship HMS Orpheus foundered and 189 sailors and marines died.

Cpt McPetrie said it was a wild, inaccessible and inhospitable shoreline.

If the Spirit of Enterprise foundered, it would have been extremely difficult to recover the crew, the ship and deal with pollution caused by fuel oil and other contaminants on board.

The MSA was investigating the grounding and has said it would also look at other incidents involving the same ship, including another grounding in Manukau Harbour in July, 2001.