The number of reports of suspected sexual offences involving children has more than doubled since 2010, the Police Service of Northern Ireland (PSNI) chief constable said.
Increased recording of sex crime - up 120% - does not equate to the same actual increase, because of past under-reporting and other factors.
George Hamilton said: "It is a sad indictment of society, but I think it is us getting a more accurate picture of where we are."
Increased reporting to police was a good thing, but the increased complexity of crime-solving posed challenges, he added.
He said: "We can only address the harm, we can only tackle the vulnerability when we know this is happening."
The chief constable said crime was becoming increasingly complex and reiterated that much of the work tackling criminals was done from behind computer screens rather than during high-visibility patrols.
He said budgets had shrunk in recent years and some official plans for the future were still in draft form because of the lack of a government at Stormont.
Policing the legacy of the past is costing millions and, while important, is not addressing the serious crime of today, he told a meeting at the Policing Board in Belfast.
There has been a 112% increase in recording of complex cyber-related crime over the last three years.
Mr Hamilton said: "All of those increasing complexities tend not to be policed by officers on the high street, in our housing estates, in high visibility jackets at 10 at night tackling anti-social behaviour.
"They tended to be researched in a back office space, in front of a computer terminal, in a suite that is properly developed to deal with victims and vulnerable people, maybe over a period of days to get some confidence in the officers and staff that they are engaging with.
"The demand has changed, the shift is going from the visible police presence to invisible police activity, but it is critical to tackle the complexity of this changing demand.
"Alongside that, we have got budgets that are going only in one direction, and it is not upward, so we have seen massive budget reductions over the last number of years."
He said public expectations around visible policing were the same if not increasing, while the nature of crime had changed.
"We need to balance this against demand which has shifted into this more complex, more labour-intensive, less visible space, that is part of the problem that we have," he added.