Problems that slowed or halted internet access for Spark customers appear to have been addressed.
Customers of the company formerly known as Telecom had problems browsing on broadband and mobile platforms for most of Saturday.
Staff worked through the night and the network appeared to be back to normal on Sunday, Spark communications head Conor Roberts says.
Some customers were still reporting problems, but that was a result of "teething problems", he told NZ Newswire.
The problem happened when overseas criminals infected a handful of customers' computers with malware, or malicious software, sending bulk traffic to offshore sites, which overloaded the network.
It was not possible to prevent another attack, the network said.
"Teams had put in place ways of managing further high volumes of traffic from this kind of thing, but this is a pretty dynamic environment," Mr Roberts said.
"The point of attack might change in the future and obviously we will work to reduce that when it does pop up again."
The affected computers had been removed from the network and Spark was working with owners to ensure they were properly protected before being reconnected.
People should ensure their virus protection and spyware was up to date and shouldn't open any suspicious links, Mr Roberts said.
"We acknowledge the impact on our customers... and apologise to those people who were affected."
Some customers vented their frustration on Facebook, while others were more sympathetic about the malware issue.
Spark changed its name from Telecom in August, promising a new era for the telecommunications giant.
