Police are appealing for further witnesses to an alleged sexual assault of a young woman in Riverton in the early hours of New Year's Day to come forward. 

Police inquiries are continuing and they are appealing for anyone who was travelling on foot or bicycle in the vicinity of Bates St and John St between 12.30am and 12.50am on January 1 to come forward. 

It comes a day after police first made a public appeal for information early yesterday. 

Now, they have renewed their calls. 

"Anyone who may have information that could assist the investigation is asked to contact Police on 105 quoting file number 220101/8606," a statement says. 

"Information can also be passed on anonymously through Crime Stoppers on 0800 555 111." 

Ahead of New Year's Eve, one of New Zealand's top detectives warned that "sexual assault is unacceptable". 

"Sexual assaults can happen to anyone at any time, but this summer police want to offer some tips for people to keep safe," says Detective Inspector Dave Kirby of the New Zealand Police National Criminal Investigation Branch. 

"It is never the victim's fault and should not be tolerated in any situation."