Diving at such depths would require a recompression chamber and the police would decide whether to call in the navy.
Eric Hertz, 58, and Katherine Hertz, 64, were travelling in their twin-engine Beechcraft Baron to Timaru to visit their daughter when they reported engine trouble about 12.20pm on Saturday, crashing shortly after into the sea near Gannet Island.
The crash site was located at 1.30pm where debris and oil was discovered.
The area was extensively searched but the two occupants were not found.
A marker buoy was placed at the scene overnight and police divers spent yesterday afternoon using sonar equipment to survey the area.
Mr Lynam said a high-impact air crash expert, together with sonar findings, indicated the couple remained trapped in the wreck.
The Coastguard had closed an area to the public to allow the mission to proceed.
Waikato Police spokesman Andrew McAlley said it was expected to be a prolonged operation.
Mr McAlley said police would decide today how to proceed with the recovery.
Yesterday, people who knew Mr Hertz or who are familiar with his work paid tribute to him.
The director of corporate affairs at 2degrees, Mat Bolland, said staff were struggling to come to terms with the loss of the company's chief executive.
"I think it's fair to say that the people at 2degrees are in shock. The fact that we won't see him back, or Kathy, is quite stunning. We're going to miss Eric's leadership, friendship and dry sense of humour.
"He was our 'honorary Kiwi' and greatly respected by our 760 staff."
During a management meeting on Thursday, Mr Bolland talked about his plans for the long weekend "and yet again Eric probably had a more exciting weekend planned than many of us".
The board and a management team are working on a plan to ensure the company continues.
Mr Hertz joined 2degrees in 2009, after leading mobile app company Zumobi in the US and with an extensive telecommunications career spanning more than 30 years.
Communications and Information Technology Minister Amy Adams said Mr Hertz was one of the true gentleman of the sector and it was a pleasure to work with him.
Ms Adams said Mr Hertz often spoke of how much he loved living in New Zealand. "He has been an integral part of the creation and growth of 2degrees, and there are hundreds of thousands of New Zealanders who are today benefiting from Eric's vision and commitment.
Actor Rhys Darby, who fronts the company's television commercials, said last night: "I'm very sad to hear about the loss of Eric and Kathy Hertz. My heart goes out to their family, in particular their daughter, Ari, and all the team at 2degrees who appreciated their success was due in part to his humour, determination and leadership.''
Gary Holmes, manager of the business association in Eden Tce, Auckland where 2degrees has a number of offices said Mr Hertz had been a big supporter of the precinct.
A US Embassy spokesman said staff were in contact with police and were actively tracking the situation.
TWIN ENGINE FAILURE 'RARE'
An aviation commentator says it is rare for a twin-engined plane to go down because of engine failure.
Peter Clark said it was vital to raise the wreckage to find out what went wrong.
"A twin-engine aircraft failing and going down so quickly, you just don't hear of that. It's really rare to have both engines fail."
"It was very good conditions and for no one to have survived the accident is very, very unusual because you would think there would have been an opportunity to have glided and landed on the water."
"This, to me, is an extremely rare case."
Mr Clark said he was surprised that Eric Hertz and his wife, Kathy, were not able to get out of the doors.
A Civil Aviation Authority investigation would check the petrol levels, petrol quality and maintenance logs to uncover what went wrong.
There is no voice or data recorder on the Beechcraft Baron plane.
"It's essential to get the aircraft out of the water, if they can, so they can begin an investigation," said Mr Clark.
