The National Cyber Security Centre has been alerted to an extortion campaign which is targeting several New Zealand organisations.
The organisations are sent an email threatening to knock them offline unless they pay up.
To demonstrate the threat is credible, the blackmailer prevents the organisation from accessing its own website for up to an hour.
The centre said it was not aware of any cases in which the organisation had been taken offline for more than an hour.
Netsafe director Martin Cocker said online blackmail threats were becoming more common for organisations and the situation would only get worse.
Mr Cocker said these kinds of criminals were now able to spread their nets wider.
"If you have to sit down as a hacker and individually code out an attack on each website, then that takes time," he said.
"But if you can automate it, as they're doing now, then you can sweep through and attack lots and lots of websites at any given time.
"Those tools are obviously developing and so more businesses are being targeted."
He said victims should contact their network security company immediately and not reply to the threat.