A man has been left with horrific injuries after being attacked by an army of fire ants while at a picnic with his young children.

Kevin Hay was at the Hidden Valley mud races in Darwin with his children Heather, 7, and Callum, 5, last month when a swarm of tropical fire ants set on him.

The invasive ants, originally from South America, have become a major pest in Australia, killing native plants and wildlife.

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A man has been left with horrific injuries after being attacked by an army of fire ants while at a picnic with his young children (stock image)

They were accidentally introduced on shipping cargo.

Mr Hay's attack was so severe it left Mr Hay unconscious, NT News reported.

'We had only just arrived. I put a blanket down for kids and the next thing I knew I had hundreds of them all over me. They were on my legs and all my arms and I was madly trying to get them off me,' Mr Hay said.

'I moved away to clean the blanket and all of a sudden I started to get dizzy and my body became weak making me drop to my knees.'

Mr Hay said he went in and out of consciousness as a policewoman and his children sought help.

After receiving emergency medical treatment Mr Hay and his children were able to go home.

But the next day the severity of his injuries became clear as his legs began to swell and pus stared to form where the ants had bitten him.

Tropical fire ants are not native to the Northern Territory and were accidentally introduced about 80 years ago.

The fire ant attack was so severe it left Mr Hay unconscious (stock image)

The ant gets its name from the burning sensation that results from its powerful sting

They are a major pest to Australia's agriculture and horticulture, according to the CSIRO.

The ant gets its name from the burning sensation that results from its powerful sting.

Bites can also result in anaphylactic shock in people allergic to wasps, ants or bees.

The ants are brown in colour and can grow up to 6 millimetres in length.