A 66-year-old man received a large cut to his head in an unprovoked attack in the Dunedin Botanic Garden, police say.

Sergeant Matthew Lee, of Dunedin, said officers were called to reports of an unprovoked attack on a 66-year-old man at the Dunedin Botanic Garden about 2.30pm yesterday.

The victim suffered a significant laceration to his forehead and the offender left the scene on a mountain bike, Sgt Lee said.

The offender and the reason for the assault were unknown to the victim.

Police were following positive lines of inquiry, Sgt Lee said.