During the attack, she was also kicked in the head and stomped on, sustaining facial and other injuries.
Mr Pritchard said the victim was traumatised by what had happened and police were continuing to provide her with the appropriate support.
She had also received medical treatment for her injuries.
Police were looking for a man in his 20s of slim to average build, with light brown skin, in relation to the incident.
The man had been wearing a dark-coloured long sleeve top, knee-length shorts, a hat and gloves, which might have been fingerless.
Mr Pritchard said as the area was popular with dog walkers and other recreational users, there was a good chance someone may have seen or heard something relevant to the investigation.
"We would like to speak to everyone who was in the general area between Harvey Norman at Melling and the Mills St area of the Hutt riverbank either just before, at or after around 1pm yesterday."
Anyone with information could contact Hutt Valley police on 04 560 2600, or anonymously via Crimestoppers on 0800 555 111.
"We also invite the person responsible to do the right thing and step forward," said Mr Pritchard.