The incident happened off Ruakura Rd near Waikato University.

Workers rushed to help a subcontractor trapped by a scaffolding tower that toppled onto him at a Hamilton work site.

The 25-year-old man fell to the ground when the temporary structure he was working on gave way on Wednesday morning.

The incident happened on a work site on farmland off Ruakura Rd, known as Waikato University No.3 Dairy.

CHRISTEL YARDLEY/FAIRFAX NZ The temporary tower was erected to allow contractors to move the conductor from one line to another.

However, the land was not owned by the university despite initial reports the person was trapped at the university campus.

The site is part of a joint project between Transpower and NZTA to deviate powerlines, making way for a new section of Waikato Expressway.

Transpower corporate communications director Rebecca Wilson said the man, who worked as a subcontractor with Electrix, was working up the tower when it collapsed.

"We had erected a temporary tower so we could transfer the conductor, and wires, across the main line.

"The temporary tower fell and we don't know why yet."

Wilson said the structure struck the fallen man, partially trapping him underneath.

"They stopped on site and suspended all work - he was freed before the emergency services arrived."

A full investigation was now underway, Wilson said, which would likely be a slow process.

"This is not a regular occurrence - our management, safety and project teams are on site to investigate."

Electrix, one of Transpower's main contractors, is employed to carry out industrial and distribution field work for the power company.

Four fire trucks and a rescue unit went to the scene but by the time they arrived the man had already been freed from the structure, Northern Fire communications shift manager Carren Larking said.

Fire crews assisted in getting the person into the ambulance but did not use rescue equipment.

He was then taken to Waikato Hospital where he was being assessed on Wednesday afternoon.