NZX is looking into share trading in the company which part-owns Northport, after details of a report recommending Auckland's port activities be moved north were published.
On Monday evening TVNZ revealed details of what it said were details of the final version of the Upper North Island Supply Chain Study.
The report, which was delivered to some Government ministers days earlier, claimed Auckland's port "was no longer economically or environmentally viable" and the Government should intervene to force activities to move to Northport in Northland if an agreement could not be reached, TVNZ reported.
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The following day shares in Marsden Maritime Holdings, which owns a number of port related assets including a 50 per cent stake in Northport, jumped 9 per cent to the highest level in at least two decades.
On Thursday, NZX confirmed the matter was being looked into.
"NZX Regulation confirms that it is assessing trading in Marsden Maritime Holdings Limited on 12 November 2019, following release by TVNZ of details relating to the outcome of the Upper North Island Supply Chain Strategy review," a spokesman for NZX said in a statement.
"NZX Regulation is not in a position to provide further comment at this time."
The Financial Markets Authority directed questions to the NZX as it was the "frontline regulator".
Marsden Maritime Holdings, which was previously known as Northland Port Corporation floated on the New Zealand Stock Exchange in 1992.
Gavin Carroll, Marsden Maritime Holding's financial controller said that he was not aware that NZX was looking at the company's trading, but was not surprised given the recent share price move.
Carroll said the company, which had not received the final version of the report, would co-operate with NZX if the exchange operator contacted it.
According to its website, Northland Regional Council owns 53.6 per cent of its shares, Ports of Auckland owns 19.9 per cent with the balance held by members of the public.
In August the company announced a net surplus for the year to June 30 of $9.1m, up 3.1 per cent on the previous year.
Earnings from Northport fell slightly, offset by an increase in earnings from the company's property and marina business segments.
Bulk cargo throughput at Northport reduced by 4.9 per cent to 3,387,000 tonnes, with log exports accounting for two thirds of volume in both financial years.
Annual container volumes increased by 61 per cent over the previous financial year.
The shares recently traded up 4c at $6.89, and have gained 30 per cent over the past year.