Waikato, Northland and the Bay of Plenty saw the biggest increases, at 26, 33 and 61 per cent, respectively.
"It's a worrying trend," said Rape Prevention Education director Dr Kim McGregor, referring to the 45 per cent rise in sex offences since 2004. "There's been no focus on sexual violence in two decades."
Frontline services had been run down since the focus shifted to family violence in the 1990s, she said.
"I've had professionals ringing today saying we need help in our community; we've got gang rape happening ... What do we do? And there's no specialist agency nearby. Some of the larger agencies are reporting up to six rapes a day coming through the door and some of the rapes are becoming more violent."
In Statistics NZ's breakdown of the sexual assaults by "scene", the vast majority were in homes. Smaller numbers were recorded at public places, shops, hospitals and others.
Sex offences at schools or other educational institutions rose from 61 to 122.
Secondary Principals Association head Patrick Walsh said the age at which children were committing sex crimes was dropping.
Schools could not be left on their own to deal with such issues, he said.
"Young students who commit sexual assault aren't created in the school environment. They often come from homes that are dysfunctional and they learn the deviant sexual behaviour in the home context."
Police Minister Anne Tolley praised police for the overall drop in reported crime last year to a 15-year low.
But there was more work to be done to "tackle crime head on", she said.
"There was a rise in recorded sexual assault and related offences, and much of this will be down to better information sharing between police and other agencies," Ms Tolley said.
"It's encouraging that more victims feel confident enough to come forward in an area which had been under-reported in the past."
