Today is what internet geeks call DNS Flag Day - and InternetNZ is warning it's set to "break" 508 internet domains - or website addresses and services associated with them, such as email.

A global change to the Domain Name System (DNS) - one of the key technologies that underpin the internet - is being rolled out, and it's expected to hit tens of thousands of websites around the world that haven't properly prepared.

Affected local websites will be offline until the issues are corrected. "It could be hours or days depending on the specific issue and the time it takes to fix it," InternetNZ group chief executive Jordan Carter says.

A phased rollout means some of the sites might not be knocked offlined until tomorrow or the day after.

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The better news is that in July last year, InternetNZ calculated that 8,349 .nz domain names would break today.

Banks and major government departments were among those whose sites would have broken.

Approaching their tech teams to get things sorted before February 1 was a priority, Carter says.

A random domain name is tested at dnsflagday.net, where you can test if a site is up to snuff.
A random domain name is tested at dnsflagday.net, where you can test if a site is up to snuff.

InternetNZ is the non-profit that administers .nz domains. It says the DNS update will introduce new features, including new mechanisms to protect against the increasing threat of DDos (distributed denial of service) attacks, which see a site forced offline as millions of bots are marshalled to try to connect to it at once, swamping its server.

"The team at InternetNZ, led by chief scientist Sebastian Castro, has put in the hard yards on this change. Being able to significantly decrease the number of broken .nz domains is a huge success for many New Zealanders."

But there are still more than 500 sites that are expected to break. If you're worried about your site, or one you access, type its address into the checker here.