Hundreds of users began reporting problems accessing the online services of ANZ, Kiwibank and NZ Post from shortly before 9am on Wednesday.
Websites for some government agencies and banks have gone offline or experienced disruption in what appear to be cyber attacks.The government's computer emergency response team (CERT NZ) confirmed there were multiplve distributed denial of service (DDoS) attacks on local organisations.
It posted: "CERT NZ is aware of a DDoS attack targeting a number of New Zealand organisations. We are monitoring the situation and are working with affected parties where we can."
New Zealand Post and Inland Revenue say the blackout is currently under investigation.
IRD's website is back online while New Zealand Post has been intermittent.
MetService was able to direct users to its backup website, which contains key information including weather watches, warnings and rain radar imagery, while it works to get its main website up and running.
ANZ Bank customers have reported being unable to log into their accounts.
Kiwibank is having intermittent issues with its website, internet banking, the app and phone banking but has identified the cause.
ANZ and Kiwibank appear to have made some progress recovering from a cyber attack that made their online services inaccessible for many New Zealanders on Wednesday.
IT security expert Daniel Ayers told Midday Report it was clear the organisations were the target of another cyber attack.
"The idea behind a denial of service attack is exactly that, to deny people access to a service.
"It's not the type of attack that's going to result in anyone's money being stolen, or that sort of fraud, but what it does do is it stops people accessing their banking or getting the weather.
"There's two possible objectives here, one is that the people are just trying to be spiteful and cause disruption and the other one is that it could be financially motivated.
"So there may potentially be a ransom going on, which is along the lines of pay us a whole lot of money otherwise we will stop you from being able to operate your business."
Large organisations generally defend DDoS attacks by using technology tools to identify and shut off the sources of the spurious traffic bombarding their services, which usually originate from networks of malware-infected computers that could be anywhere in the world.
But because attacks can often be directed through legitimate but poorly-configured web servers, it is not generally straightforward to find the exact source of such attacks.
Sometimes attacks stop, only to restart from a different source.
Commonly, attackers demand ransoms to stop their attacks, though it is believed these are rarely paid.