An investigation into NSW Police has been launched after a man died and another was left in a serious condition after an alleged brawl.

A 31-year-old man with serious head injuries was taken to Lismore Base Hospital in northern New South Wales on Friday morning by an unknown person.

He was airlifted to Gold Coast University Hospital and remains in a serious condition.

Later that morning, police found the body of a 25-year-old man on a walking track near Fischer St, Goonellabah in Lismore.

It is believed both men were involved in a brawl at Clifford Park in the suburb that morning alongside 15 to 20 other people.

A critical incident has been declared to analyse the response of police to the initial fight.

The investigation is being conducted by officers from the Mid North Coast Police

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