A Waikato woman lost her arm after a farming incident.

Exactly how it happened remains unclear, but a spokeswoman for the Waikato Westpac Rescue Helicopter said the chopper was sent to a farm, east of Matamata on Friday afternoon to retrieve the 58-year-old.

It is understood the woman's arm got caught in machinery.

She "suffered an amputation of her left arm, at the elbow".

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She was taken in a critical condition to Waikato Hospital.

Accompanied by her partner, she was flown to Waikato Hospital for treatment.