The search for the privately owned Robinson R22 helicopter was called off late last night, after searchers using night-vision equipment found no trace of the aircraft.
It was last heard from about 12.45pm yesterday, while on a training flight in the Awarua area, near Invercargill, Inspector Olaf Jensen, of Invercargill, said.
The helicopter - with an instructor and trainee on board - was reported overdue about 2.15pm, but no emergency beacon was activated and there was no further communication with the helicopter.
The missing pair on board were from Southland but were not associated with a flight school, Insp Jensen said.
No other details about the occupants were available last night.
The search was being led by Rescue Co-ordination Centre New Zealand (RCCNZ) staff.
Up to five helicopters were involved in sweeping a 1250sq km area east of Bluff, covering land and sea, Maritime New Zealand spokesman Nick Churchouse said.
It was not known where the helicopter had come from or planned to land, or whether - if it had crashed - it had come down in the sea or on land.
"Your guess is as good as mine," he said.
"That's the problem they're dealing with.
"No-one knows where it's gone down, so we're not ruling anything out."
The Otago Regional Rescue Helicopter, piloted by Graeme Gale, of Dunedin, was among those searching.
A fixed-wing Coastguard air patrol aircraft and Coastguard vessels from Riverton and Bluff were assisting in the search.
Maritime New Zealand had also appealed for any vessels in the area to look out for sightings of the chopper.
Insp Jensen said searchers had few clues to the helicopter's possible location.
"We are searching the area where we presume the pilot was intending to go into training, but we have no definite setting or area," he said.
"We have limited information to go on."
The Awarua area was a wetland and designated training area, and conditions at the time the helicopter vanished were good, he said.
"It was a perfect day here - perfect flying conditions," he said.
He declined to release the pair's names or other details last night, saying the police were still dealing with their families.
Riverton Coastguard skipper Noel Anderson said the Coastguard had been conducting a grid search but had seen no sign of the missing machine or its occupants.
"Nothing's been sighted yet."
Mr Churchouse said searchers using night-vision equipment planned to make one more pass late last night, before standing down for the night.
RCCNZ staff would review search options overnight and plan for the search to resume at daybreak, he said.
