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But Warrant Officer James Harper - officer in charge of the Navy's Mine Countermeasures team - said he was taking confidence from the progress of a five-hour, 1km-square sonar scan of the ocean floor this afternoon (Mon).
Mr Hertz, 58, and his wife, 64, were flying to Timaru to visit their daughter on Saturday. During the flight he reported engine trouble and ditched the plane into the sea.
"Everything is based on the best information you get at the time," Warrant Officer Harper told a media conference in Raglan today.
A 20-strong team of Navy, police and Coastguard staff and volunteers were doing their "utmost" to retrieve the bodies, believed to be in the wreckage 60 metres below the surface.
Sea conditions were fair today, with swells of around 2m.
"Can I put a confidence factor on it? Not necessarily now, but I have a reasonable confidence level in what we're achieving today."
The sonar device being used captured data that would be analysed on shore.
If the wreckage wasn't found, the search area would be widened tomorrow.
Sergeant Warren Shaw said it was hoped the wreckage would be located today but even if its position was found on the ocean floor it could be at the limits of the depth divers could reach - around 60 metres below the sea surface.
"Police are thus relying heavily on the expertise of the Navy and their experience in this field," he said.
"One of the major challenges we're facing is that we're dealing with an aircraft that has potentially impacted with the water at high speed and broken up.
"To compound things further, there's the potential for objects from the wreckage to drift up to 5km a day due to ... currents."
A buoy was dropped at the scene where debris from the plane and an oil slick were discovered, about 20km northwest of the entrance to Kawhia Harbour.
An aerial search today was also scanning for debris on the shoreline, which may indicate drift patterns.