Internet New Zealand says it's up to customers to secure their devices against malware attacks.
Spark says service on its broadband network has returned to normal after malicious computer programs bombarded the company's servers and restricted internet access.
Internet NZ's deputy chief executive believes Spark was the "likely target" because it has the largest customer base in the country. Andrew Cushen told ONE News the attacks work best when hackers can get the most people infected to "create the most amount of havoc and damage".
"This was a high profile incident...these people were out to make an impact through causing the most damage possible."
Mr Cushen says companies can't secure against these types of attacks but users can secure themselves.
"This is in the customers' hands...they must be vigilant, have up to date security settings and anti-virus software."
And Spark agrees with Mr Cushen that these attacks are likely to become more common over time.
Jason Mangan, head of service delivery, said Spark has put mitigation activity in place but attacks like these are dynamic in nature and take different forms.
"Attacks like these happen frequently but the level of this one is unprecedented," Mr Mangan told ONE News.
Warnings about new malware attacks, linked to the recently leaked photos of naked celebrities, have been issued by internet security companies in the past few days. Spark says it can't be sure whether the problem is connected to these scams, but it did happen around the same time.
Mr Mangan said Spark is investigating all avenues to "counter attack, prevent or mitigate" but is not downing tools because another attack of a different nature is possible.