The Rena's passage plans were repeatedly changed on its journey from Napier to Tauranga on October 4 and 5 last year, a Transport Accident Investigation Commission (TAIC) interim report reveals.

The captain of the container ship was looking out at the ocean trying to see what a radar signal had picked up moments before the Rena crashed into the Astrolabe Reef.

The ship was heading directly for the reef, at a speed of 17 knots, when the radar signal alert was activated, just nine minutes before the grounding.

But the master wasn't aware of the course the ship was on and was walking back to plot the ship's position when it grounded.

The commission released its findings this morning, which show that the Filipino crew took several shortcuts so they could reach the pilot boat near Tauranga by 3am on October 5.

But they never made it to their destination - the ship grounded on the reef at 2.14am.

The report shows that the ship's position was recorded on two different charts, which were not cross-referenced; that there was a 2 degree difference in the ground tracking (the vessel's track) and the compass reading; and that documents were altered after the grounding.

Statements from the second mate and master about what happened in the moments leading up to the grounding contradicted each other.

While the second mate told investigators that he showed the master the charts, plotting the ship's path, prior to the radar signal alert, the master said he didn't see them.

Commissioners boarded the ship on October 6 and took documents and equipment to analyse.

The documents reveal that the last time the ship's position was marked on a chart was at 1.20am, which showed the vessel starting to divert from its plan and head towards the reef.

The Tauranga Harbour Control had given the Rena a deadline of 3am to reach the port pilot which would steer the ship into the port.

The second mate called the master at 1.35am to discuss altering the course to shorten the journey. The master agreed, but documents show that the ship had already started to divert from its passage plan at 1.20am, 15 minutes before the call was made.

"The second mate's plan was to navigate closer to Astrolabe Reef," the report states.

"He said he placed a mark on the chart about one nautical mile north of Astrolabe Reef, which was the point to which he intended to navigate the Rena before making the final course adjustment to the pilot station. The master has stated the mark made on the chart was put there after the grounding."

A watchkeeper later went to the chartroom to plot the ship's position at 2am, but the master and second mate were leaning over the chart and he didn't want to interrupt them.

He noted the position in the log book, but someone later altered it.

The ship's position at 2am was plotted after the grounding, and was further north than the Rena's position at the time, TAIC's investigator in charge, Robert Thompson, said.

But it is still unclear as to who was responsible for making the mark, Thompson said.

There was no evidence to suggest that the crew were drinking or celebrating the master's birthday, as rumours had suggested, Thompson said.

But the culture on board the ship and the relationship between management and crew would be investigated.

The ship's passage plan was altered several times throughout the course of its journey from Napier, including a shortcut around Mahia Peninsula, which saw the ship travel a lot closer to the peninsula than originally planned.

The ship was steered by autopilot for most of the voyage and from midnight until the grounding.

Thompson said it was "normal" for a ship's course to differ from the grounding track, as waves, currents, tide and the wind will move the ship's course sideways "so the direction the vessel is heading in may be different to the ground track".

The second mate had set up a parallel index and used Motiti Island as a reference. The index would be compared with the ship's course, but the master turned it off because it was cluttering the system. Parallel indexes are not mandatory, Thompson said.

More details will emerge as the investigation continues, chief investigator Tim Burfoot said.

Burfoot said the crew were aware of the Astrolabe Reef as the ship's original plan reveals. But a series of small alterations and the two degree difference between the ground tracking and compass reading led the ship straight into the reef.

"Our purpose is not to lay blame and we don't assist any agency whose responsibility is to lay blame," Burfoot said.

The investigation's findings cannot be used in civil or criminal proceedings, but will be used as a basis to improve maritime safety.

Both the master and second mate have pleaded guilty to charges over the grounding. They have remained in the country since the grounding and are awaiting sentencing.

MARITIME NZ RESPONDS

Maritime NZ said it would consider the report in due course, but chairman David Ledson said the majority of accidents were caused by human error.

Earlier, Ledson acknowledged Maritime NZ had failed to communicate with the public in the early days of the Rena grounding and had been reluctant to use volunteers.

He told Parliament's transport and industrial relations select committee his organisation had learnt valuable lessons from the Rena incident.

Maritime NZ was also slow to engage with local iwi, he said.

The emphasis had been on a technical approach but there also needed to be more empathy with the community.

"There was a threat to the environment which made it important (but) there was always going to be an emotional response."

From the beginning Maritime NZ's response was based on the belief an oil response was needed and they knew best how to handle that, Ledson said.

"But there was another part of it too which was called 'a ship on a reef'."

Things happened reasonably fast in those first few days but the communication with the public did not properly express that, he said.

However, by reflecting on the process as things went along they were able to adapt and improve the relationship with those affected.

"We learned that it's not just a matter of speaking at the community. You've actually got to engage in conversation with them."

Best practice in New Zealand was to engage with volunteers, he said.

No decision had been made on whether to prosecute the owners of the ship, but the fall-out from the grounding had cost Government $30-$35m so far.

The removal of the containers would be a "long, slow, demanding process", chief executive Keith Manch said.

It was likely to take 4-6 months and then it would be decided what to do with the remainder of the ship.

Labour environment spokesman Grant Robertson said the report released today highlighted the need for a full inquiry.

"While the series of mistakes that led to the ships grounding are important, investigating the timeliness and adequacy of the government response and what changes are needed in the event of a future incident are equally urgent."

RENA: WHAT WENT WRONG

* Rena under pressure to make 3am pilot boat deadline off Tauranga

* Rena course changes to short-cut the distance to the pilot boat takes it toward Astrolabe Reef

* Confusion over the new course on the bridge, with the watchkeeper, captain and second mate all involved

* two navigation systems gave different readings, two degrees apart

* the crew failed to immediately react to a 2.05am echo signal dead ahead on the radar

* conflicting evidence over when a pencil mark on the chart one nautical mile north of Astrolabe was drawn

* Rena runs aground at 2.14am, travelling at a speed of 17 knots (31.5kmh)

- © Fairfax NZ News