Police believe he ran back to his van at the summit and then drove away.
A member of the public saw the woman was upset and called police.
Officers searched the area but the man was not found.
Detective Keri Kimpton said police needed to speak to anyone who was in the area at the time.
"Mt Wellington is a reasonably busy place on a Sunday morning, lots of people walk and run up there at that time and often take selfies or photos at the top.
"We'd like to hear from anyone who was there around the time that this occurred, particularly anyone who took photos, to get in touch with us please."
Kimpton did not believe the incident was linked to the ongoing investigation of indecent assaults against women in Onehunga.
"Any incident like this is alarming and we are obviously very keen to catch this person," Kimpton said.
"We'd also like to hear from anyone who has seen someone of that description behaving suspiciously at another time on Mt Wellington."
The offender is described as the following;
• Aged 25-30 years old.
• Maori or Pacific Islander.
• Was wearing shorts.
• About 170-172 cm tall.
• Dark/black curly hair, about collar length.
• Slight moustache and stubble on cheeks.
Anyone with information is asked to contact Detective Keri Kimpton on 09 524 1921.
Information can also be given anonymously to the organisation Crimestoppers on 0800 555 111.
