A search by the Police Eagle Helicopter, a dog unit and ground patrols did not locate the offenders.
The victim, who is five months pregnant, was shaken but not physically harmed, and was taken to hospital for a check-up.
The men were described as:
1. A Maori or Pacific Islander aged his late 20s, with a solid/muscular build. He is described as wearing a black hoody, dark jeans with rips in the legs and dark-coloured sandshoes.
2. A Caucasian aged in his late 20s, around 165cm tall, with a thin build. He is described as wearing a baggy red hoody, faded low-cut jeans and possibly brown sand shoes.
Police would like to speak with anyone who was in the Piha Road/Upper Nihotupo Reservoir area at the time and saw anyone fitting the description, or behaviour which appeared suspicious.
Anyone with information is asked to call the Waitakere Police on 09 839 0629.
Information can also be given anonymously to Crimestoppers on 0800 555 111.