A spike in crime committed by women and youths has left the head of the police wondering if social media may be partly to blame.

Answering questions about crime rates from Parliament's justice select committee on Thursday, Commissioner Mike Bush said he was most concerned about a growth in young women involved in crime.

"Whether it's in the aggregated robberies that we're all concerned about, or in general crime - and this is reflected in the prison population - there is an increase in women, young women, being involved in crime," he said.

While the overall number of crimes among youths in general was not trending upwards, the nature of offences being offended by teens was becoming more serious, Bush said.

Asked about whether police had any ideas about the cause, he said it was being studied - but wondered if social media didn't have to carry part of the blame.

"One of the things that does concern me about crime is some of the motivation is actually about people getting notoriety," he told reporters.

"On social media people will upload their activities quite quickly thereafter. My concern that some of these crimes are driven out of wanting a few minutes of fame."

He was unable to immediately provide exact figures on the number of young women committing crimes. Those statistics have been requested from the police.

Overall, Bush described the last few years as "challenging" for police in terms of crime rates, but said the backwards trend had last year turned.

"Yes it's only a 1 per cent drop, but it is a drop," he said.

"In some areas, in the serious crime categories ... the drop is more significant most recently."

- NZN