Search teams spent a second day yesterday in the Haast River area looking for missing Canadians Connor Hayes (25) and Joanna Lam (24).
The pair were last seen at Fox Glacier last Tuesday, before their rental van was hurled down a remote gorge in a landslip.
The vehicle was found smashed to pieces at the bottom of an 80m gorge on the Haast Pass road, which was battered by heavy rain and landslips during a massive storm last week.
A fuel tank belonging to the wrecked campervan was found washed up on a coastal beach about 50km from where the vehicle left the road, and helicopters on Wednesday spotted one of the van's seats.
Police said the van was probably pushed into the gorge by a landslip on State Highway 6, which was closed by another, huge slip nearby.
A helicopter with heat-seeking equipment helped in the search yesterday, as well as a specialist search dog on the ground.
Constable Rob Manera, of Haast, said search crews wanted to ''exhaust every possibility, do everything we possibly can'' in the search to find the couple.
''But the reality is ... there's been that much gravel that's been flushed out of that gorge and down that river in that flood ... hundreds of thousands of tonnes of gravel, so stuff got buried, so there's only so much you can do.''
More debris had been discovered washed up on beaches after being swept out to sea, so it was also possible the same had happened to the bodies of the tourists.
''Tomorrow's plan is to do the beach again on motorbikes and use the police cadaver dog along the build-up of driftwood between Haast and Ship Creek.''
West Coast area commander Inspector John Canning has conceded the chances of finding the pair alive are extremely remote.
''Our effort now is a recovery operation and, weather permitting, we will continue searching until we have exhausted all reasonable possibility of finding these people.''
Deteriorating weather in the Haast area yesterday limited the search. However, more debris from the wrecked vehicle was found, Insp Canning said.
The van's chassis, found wrapped around a large boulder in the Haast River on Tuesday, has now been retrieved from the river.
Any future searches would be dictated by the weather, which was expected to improve by Sunday, Insp Canning said.
The New Zealand Transport Agency said yesterday it would review its road closure procedure when roads were under threat by bad weather.
SH6, from Haast township to Makarora, was officially closed at 8pm on Tuesday last week.
''Also on Tuesday afternoon, there were cautionary warning signs on display between Fox Glacier and Haast,'' NZTA acting media manager Ewart Barnsley said.
Contractors inspected the highway about 9.30pm that night.
''The large slip at Diana Falls was discovered on Wednesday morning by contractors.''
NZTA said it hoped to reopen one lane of the road from about noon today.
However, rain could close the highway at short notice due to an increased risk of more debris coming down from the Diana Falls slip.
NZTA Canterbury/West Coast regional performance manager Pete Connors said highway openings would be restricted to daylight hours between 8am and 6pm and managed carefully until the slip settled.
- APNZ/Additional reporting Lucy Ibbotson