An eyewitness has described seeing a man "on fire" after an explosion at a Vector transformer outside a South Auckland factory.

The man, a Northpower technician, received serious electrical burns after the transformer blew near the site of the Frucor Suntory plant in South Auckland.

The witness said the explosion sounded like a shipping container had been dropped, and power was knocked out at the soft drink maker's factory.

They then saw a man "on fire" walking to retrieve his phone, before realising his skin was starting to peel.

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Emergency services were called to the commercial address, on Orb Ave in Manukau, just before 1pm.

Vector spokeswoman Minoru Frederiksens said the company was aware of the incident on its network.

"Worksafe has been notified. We are taking every step to ensure the individual involved receives the appropriate level of care and support. We will carry out a full investigation to determine the facts of the incident. Until then we have no further comment."

No Frucor employees were injured in the incident, according to a spokeswoman for the soft drink maker. She said power had been knocked out to one building at the Frucor site.

A Fire and Emergency NZ spokesman said one person had sustained electrical burns to their head and hands after a transformer caught fire.

A St John spokeswoman said two ambulances attended and transported one patient in a serious condition to Middlemore Hospital.

Frucor Suntory is a leading Australasian drinks company and the market leader in energy drinks in Australia and New Zealand.