Former AFL star Michael Long had acted like "any red-blooded" man when he assaulted a man who had earlier attacked his sister, a magistrate said.

Long has escaped without a conviction after pleading guilty in Darwin Magistrates Court to one charge of assault, and his only penalty was an order to pay a mandatory $40 victims of crime levy.

The 37-year-old former Essendon premiership player was accused of striking David Henry at St Mary's Football Club after the Northern Territory Football League grand final in March.

Long was angry because he was told the man had attacked his sister in the past. He yelled at the man and punched him three times, the court heard.

Magistrate David Loadman said Long had acted in the heat of the moment after coming across Mr Henry.

He said Long had acted as any "red-blooded man" would do to defend his sister and that he was glad there still were such men around.

The magistrate described Long as a football "legend" and said he had taken into account his good character and contribution to the community, especially in working against racism in sport, as well as the circumstances of the alleged assault.

Mr Loadman did not record a conviction, or a bond, saying that information and testimonials he had been handed indicated that Long was unlikely to re-offend.

"I hope today will raise the awareness of such violence against women," Long said outside the court.

Long won the Norm Smith Medal following Essendon's 1993 grand final victory over Carlton.

He made headlines in 1995 when he accused Collingwood's Damien Monkhorst of racially abusing him, prompting the AFL to introduce a racial vilification policy.

Long retired at the end of the 2001 season but has continued to raise awareness of the plight of indigenous Australians.