The young woman described the offender as dark-skinned, possibly in his early 20s, wearing a khaki-coloured top with a hood and a black beanie.
A scene examination was likely to be completed later today and an area canvas was underway.
Mr Basham said police would like to hear from anyone in Wellington's eastern suburbs who may have seen or heard or seen anyone or anything suspicious last night.
The assault is being treated as an isolated incident and police are urging Seatoun residents not to be alarmed.
"Police want to reassure the Seatoun community that this incident is being treated seriously and a number of staff and resources are being put into locating this offender, which, with the help of the public, we are very confident of doing."
Mr Basham reminded people to take the usual safety precautions.