In a DDoS attack the hackers overwhelm a site with thousands or millions of bots trying to connect to it at once, rendering it inaccessible. There's no element of breaking into servers or stealing data.
In an update at 8.30am on its Facebook page Kiwibank said it was expecting its internet banking and app to be intermittent today.
"Some customers may be able to access our services and some may have issues from time to time. We're continuing to work on this as our priority."
The bank said it knew the outages had caused challenges and frustration for its customers.
"We really do appreciate the patience and support you have shown while we work around the clock to get our services back up and running."
It also urged customers to plan ahead if they needed to make a payment or transfer money using internet banking or its app.
But one customer said he planned to switch banks as soon as the systems were up and running again.
"My business doesn't allow time to sit and wait until these constant issues are sorted."
Others complained about having their card payments declined while trying to make purchases.
"I wasn't able to purchase my groceries last night. Was very embarrassing."
Another customer questioned whether the bank would provide refunds for dishonour fees.
"Hope your reimbursing customers who have been charged dishonour fees due to not being able to transfer money over to go out for payments."
The bank said it would reverse any fees its customers had incurred due to the outages including phone banking and dishonour fees.
"Any fees that may have been charged will be refunded."
"Our contact centre and social media team are here to help today as needed though please expect longer wait times if needing to call us."
Last week ANZ experienced three days of outages in a row on Wednesday, Thursday and Friday.
Asked if the Reserve Bank was concerned about the attacks on the two major banks a spokesman said: "We do not comment on the operational matters of individual entities regulated by the Reserve Bank of New Zealand - Te Pūtea Matua.
"We are in contact with the affected entities and they are keeping us informed of their developments."
Last Wednesday Government cyber security agency CERT NZ said it was aware of a DDoS (Distributed Denial of Service) targeting a number of New Zealand organisations.
"We are monitoring the situation and are working with affected parties where we can."
A spokesman for the GCSB's National Cyber Security Centre said, "We are limited in any public comment we will make as we are aware that malicious cyber actors can follow what is reported publicly, and may change their behaviour based on media reporting of their activity.
"DDoS attacks are not new, and most are repelled by organisations working with their service providers who are best placed to implement technical mitigations."
