Lam, 24, and Hayes, 25, were last seen at Fox Glacier on the afternoon of Sept. 10, according to police.
On Tuesday, officers found parts of their wrecked rental van in a gorge in a remote park on the South Island.
They heightened their search Wednesday with a heat--seeking helicopter and a specialist search dog, in addition to a search and rescue boat.
In a statement, Insp. John Canning said police have now declared the search a recovery mission.
"Weather permitting, we will continue searching until we have exhausted all reasonable possibility of finding these people," the statement read.
- Missing Ottawa couple's wrecked van found in New Zealand
The search has been limited by "deteriorating" weather, it added, but police did find more debris from the wrecked van.
Also on Tuesday, the rear wheels and chassis were located about 80 metres down a bluff in the Haast River. The van's body and engine have not yet been located.
Police don't yet know whether Lam and Hayes were inside the vehicle when it left the road.
New Zealand police have said the van was likely swept away in fast-moving floodwaters, as the area where it was found experienced a bad storm more than a week ago.
Ceinwen Curtis, a reporter with Radio New Zealand in Christchurch who is covering the search, has also said there have been a number of landslides at that mountain pass close to where the couple's vehicle was found.
She said boulders as big as houses remain on the road, a week after the storm.
A friend told CBC News earlier this week Lam failed to show up at her new job as a medical imaging specialist for a radiology department in New Zealand on Monday.
Tamara Sagadore said Lam and Hayes have been dating for almost two years after meeting and living together in Ottawa.
Hayes, whose family is from the Toronto area, was said to be passionate about running. Lam grew up in Kingston, Ont., where her family still lives.Suggest a correction