A woman has lost an arm in a machinery accident on a Waikato farm.

Emergency services were called to the incident east of Matamata at about 5pm on Friday.

The 58-year-old woman had become trapped in machinery and was found in a critical condition.

A Waikato Westpac Rescue Helicopter spokesperson says the woman suffered "an amputation of her left arm at the elbow".

The woman was flown to Waikato Hospital.