Steve O'Connor, a former exploration manager for FCE subsidiary Petrocorp, told the court late last week that his independent assessment of the Mangahewa structure was a minimum mean unrisked reserves of 572 billion cubic feet of gas.

His figure was almost 10 times the reserves number, of 67.5Bcf, used by Sydney-based consultants Nicholas Papalia & Associates for FCE's takeover of Southern, which appeared to be based on a single well.

O'Connor said his analysis was a "prospect-specific" one over the whole Mangahewa structure, as compared to the apparent Eocene-aged Kapuni formation generic model used by NPA.

O'Connor also said that FCE had told methanol manufacturer Methanex Corporation in mid-November 1995 that it believed the whole greater Mangahewa structure could contain up to 7.65 trillion cubic feet of gas.

A week before O'Connor gave testimony, another former Petrocorp exploration manager, Tom Haskell, told the court he believed a deep gas study done for Petrocorp, which included an area covering the now commercial Pohokura, Mangahewa, Turangi and Kowhai fields, showed that these fields could be linked.

Haskell said the subsequent near-shore Pohokura and onshore Turangi and Kowhai gas finds pointed to the entire structure containing more than 7 trillion cubic feet (Tcf) of gas.

The offshore Taranaki Maui field was originally believed to hold about 4Tcf of gas.

Haskell said had he been aware of the extent and results of the Mangahewa study at the time of the FCE takeover, he would have recommended to Southern shareholder John Oakley that he negotiate with Southern Petroleum on the basis of $NZ11.22 per share. Instead investors were offered a share price of 63c.

Minority shareholders of Southern Petroleum are seeking about $NZ23 million (about $A18.8 million) in compensation from Shell Exploration New Zealand.

The shareholders argue that FCE was aware of the results of the deep gas study but withheld information about the potential of the area, which included Southern's stake in Mangahewa and in the exploration licence PEP 38459 that later yielded the 1Tcf-plus Pohokura gas discovery.

They also claim that former FCE and Southern Petroleum director Jim Patek was at a presentation on the deep gas study held at New Plymouth in November 1995 and that the results of that study prompted Patek to push through FCE's compulsory purchase of Southern.

Shell, which bought FCE's New Zealand assets in 2001, denies any liability. It is also paying the costs of the High Court case, which is being heard in Auckland by Justice High Williams and may not finish until early June.