An internationally recognised marine risk expert has plotted the fateful final hours of the ship's course using data transmitted by the ship's own instruments.

The Dominion Post has obtained charts made by a Wellington-based senior partner in Marico Marine, John Riding, who said if someone looked at the ship's charts after changing course the reef would be "staring them in the face".

"They have deviated from their planned course, and a watch keeper would not have a reason to make a decision to deviate," Mr Riding said.

"You can see they've made that managed change of course and kept everything steady."

The charts Mr Riding made of the Rena's course and speed were made using specialist technology and software developed by Marico Marine. The technology receives coded transponder transmissions that the ship's own instruments must transmit.

Another company, Teamtalk, delivers the data from remote reception sites throughout New Zealand and data – containing the ship's identification, speed, course and GPS location – is decoded live.

The data is then plotted on electronic navigation charts to show exactly where the ship went and exactly what speed it was doing at Greenwich mean time.

Mr Riding said the change of direction saw the Rena heading straight towards the first harbour entrance marker – a radar beacon close to where a Port of Tauranga pilot would take the ship into port.

"So they've made radar contact and they know the pilot's boarding position is close to it [the beacon]. They've changed course to go to the beacon.

"From when they started to alter course they had at least an hour to recognise the hazard and it would have been staring them in the face on their chart."

Mr Riding said the Rena began to accelerate and hit about 33 kilometres per hour (17.8 knots), close to its top speed, as the ship got into sheltered waters, ironically provided by the reef itself.

Warning lights from Motiti Island would have been visible.