About 70 members of the gang arrived in Queenstown on Sunday after a national hui in Christchurch last weekend.
Members had been seen walking around town, visiting bars and restaurants, taking jet-boat rides and visiting a car show.
Sergeant Simon Matheson, of Queenstown, said members of the gang were "passing through Queenstown to another location further south" after their gathering in Christchurch.
"We are aware of their presence and have had good engagement with them," Matheson said.
Mongrel Mob members in Amberley on January 29. Photo / George Heard
Police in Christchurch said a Mongrel Mob member was involved in an unprovoked attack on Oxford Terrace - a busy night spot alongside the Avon River - about 1am on Sunday.
Detective Senior Sergeant Nicola Reeves said a man punched another man, causing the victim to fall to the ground and hit his head on the pavement.
"The victim became unconscious and was transported to hospital where he remains."
