Police have asked the Navy to recover a helicopter that crashed into Lake Sumner in North Canterbury on Monday.
Pilot Michael Graeme Mehrtens, 35, is feared dead.
Police are now reasonably confident they know the approximate location of the helicopter, based on an oil slick and debris found so far.
Inspector Craig McKay says the lake is up to 125 metres deep, which is well beyond the level at which the police dive squad can operate.
The search has now been suspended until further assistance is available.
Mr McKay says a possible option would be to use a submersible remote-operated vehicle equipped with a grappling hook, which could be used to move the helicopter to shallower water.