The students are aged 16 and 17 years and are in Year 12.
The incident comes as heavy rains sweep down the North Island causing severe flooding in drought afflicted South Waikato.
Inspector Paul Jermy said the students were swept away in the Mangatepopo River by rising water levels and conditions in the area were hampering the search.
The students were part of a group of 40 Elim students attending a week-long outdoor education course at the Sir Edmund Hillary Outdoor Pursuits Centre (OPC) in Turangi.
Elim principal Murray Burton said tonight the students were canyoning down a river near the centre when they became separated from the main party.
They did not show up to be collected along with other students at the end of the activity late this afternoon.
Mr Burton was informed they were missing about 5.50pm.
"From what I understand they were well equipped for the journey with wetsuits, life jackets and harnesses," he told told NZPA.
"It is a fairly standard sort of activity. I guess it was the OPC's call as to whether they should still go and I have no reason to doubt their judgment."
Inspector Steve Mastrovich from Taumarunui police said a helicopter swept the river before dark and a helicopter with night vision capability was travelling to the area to help with the search.
Search and rescue teams from Wanganui and Taupo are in the area searching.
The grandfather of one Auckland student attending the outdoor pursuit centre this week said he understood the students from different schools were split into four groups and one group were missing.
"They'd been in camp for three or four days and today a river trip was supposed to have been the highlight," he told NZPA.
He said his family were waiting anxiously to find out from police who the missing students were.
Department of Conservation spokesman Dave Conley said tonight the Mangatepopo, which runs off Mt Ruapehu, was not a river that was in any way traditionally used for commercial activity."
A number of tracks accessed the river but they were not well known or marked. The river ran past a site used by the outdoor pursuits centre.
There were no Department of Conservation tracks managed on the river, he said.
Earlier today a man was killed and six other people ended up in hospital after they were struck by lightning on a farm west of Dargaville.
The 51 year old Auckland man was riding with the Northland Hunt group, participating in a week long hunt at Hilliam Rd, Mahuta, near Dargaville's west coast.
Northland Hunt spokesperson Donna Austin says the group were riding in heavy rain at around 12.30pm today when an electrical storm started.
"It was the most bizarre weather patterns, forked lightening, the lot and that is when the gentleman was struck and killed and so was his horse," she says.
Ms Austin says six other people were also struck but were not injured.
They were taken to Whangarei Hospital and are being kept overnight for observation.
She says the group were spread out, and those struck were on a hill.
The injured riders were attended to within minutes by two GPs, three registered nurses and an advanced paramedic.
"This weather was totally unexpected. I have never seen anything like this situation before and I have been doing this for over 20 years," Ms Austin says.
Most of the group have decided to continue with the hunt, which started on Sunday.
"We have four organised rides throughout the week and the man who died is a well respected member of this group and I'm sure that's what he would have wanted us to do.
"This is just the most bizarre tragedy and it will take everyone some time to get over it.
A band of thunderstorms are moving down the country and police say severe flooding is hitting the South Waikato area and the Western Bay of Plenty.
State Highway One on the south side of Tirau has severe flooding and is down to one lane.
State Highway 29 in the lower Kaimais on the Tauranga side also has severe flooding.
The rain will reach down into the central plateau, which should go some way toward replenishing Lake Taupo, which late last week was at one of the lowest levels recorded and close to affecting power generation.
We were just phoned up by a family friend and told the news as we know some of the people involved, and the Elim school is just down the road from us- checked the stuff.co.nz newspage and sure enough, it was the website headlines. I hope and pray everyone involved turns up safe and sound!
