A woman crushed by a one-tonne piece of farming equipment was "incredibly stoic and brave" an attending paramedic says.

The woman, believed to be in her 40s, sustained serious injuries after being crushed under an irrigation tool at a farm in Tuakau, South Auckland on Thursday afternoon.

She was in stable condition at Middlemore Hospital this morning.

Westpac rescue helicopter intensive care paramedic Chris Deacon said the irrigator, held by two wheels and carrying heavy piping, appeared to have tipped or fallen on to the woman.

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"It was clearly an accident," he said.

"It looks like it's just tipped up and fallen on to her."

The woman had been moved from underneath the irrigator by the time emergency services arrived and placed in the back of a van, he said.

The woman's husband was on the scene and everybody seemed calm but the woman appeared to be in a lot of pain.

Deacon believed she was a farm worker.

She had been given some pain relief by the time the helicopter arrived and was conscious and able to answer questions.

"She was incredibly stoic, she's very brave," Deacon said.

The woman was transported by helicopter with serious injuries to Middlemore Hospital.

He couldn't elaborate on the nature of her injuries to protect her privacy.

A Middlemore Hospital spokeswoman said the woman had undergone surgery last night and was in a stable condition.

A WorkSafe spokesperson said they had received no notification about the incident.