A cable on the Hopu Ruahine bridge on the Lake Waikaremoana Track "released" about 1pm on Thursday - causing four French tourists who were on the bridge at the time to fall into the river below.
The group survived without major injury.
Te Urewera Board chairman Tamati Kruger said it was thankful the "shaken" trampers were not seriously hurt.
The board had convened a response team to the incident, consisting of Te Urewera Board members and the Department of Conservation.
It has already ensured all six of the other "critical structures" on the Lake Waikaremoana track were safe and would investigate the fault which caused the collapse, he said.
A WorkSafe NZ review as well as an independent Health and Safety review would be carried out. It should take about 10 days to complete the reviews.
"The board, Tuhoe and DoC are taking the incident extremely seriously," Mr Kruger said.
"We will continue to want to know how our trampers are in the days and weeks that follow. We are planning to inform them of the investigation outcome once complete."
Department of Conservation operations manager Mike Slater said the incident could have had a much more serious outcome.
"As a result [the released cable] has upset the stability of the bridge and that's where people that were on the bridge have slipped off and fallen.
"Fortunately other than some scratches and a little bit of bruising, they are not seriously injured. That's a very fortunate outcome."
The bridges are inspected by an engineer every six years and by a qualified departmental inspector every second year. Every 12 years they undertake a full load testing, he said.
The bridge was last inspected in September last year.
"An event such as this is [an] extremely rare occurrence and we just need to understand in this particular case exactly what did happen."
The bridge was now closed, Mr Slater said.
The Lake Waikaremoana Track, in the Te Urewera National Park in the central North Island, is one of DoC's nine Great Walks.
The three- to four-day tramp follows roughly half of the lake's circumference and is a popular holiday destination.
It remains open from Onepoto Rd to Whanganui Hut and is to be walked in only one direction from Onepoto until further notice. Water taxi pick-ups have been arranged from the Whanganui Hut at 2pm daily.
Contact Te Urewera Visitor Centre for more information.
