"At this early stage of the investigation, it appears the two parties involved are not known to each other.
"This is an area of focus for the investigation team."
Police said there were a number of people in and around the Waikanae Train Station, who may have seen the assault or the offender.
They said there was also a possibility the offender got on a southbound train shortly after the attack.
Police are appealing for information from anyone who may have witnessed the attack, seen the offender, or know the offender's identity.
Detctive Senior Sergeant Ferguson said, "This brazen violence in a public place is of real concern to us, and we are committed to responding quickly to ensure the public's safety, and preventing further incidents."