Internet users were reporting problems accessing a range of websites including those of ASB and Westpac on Thursday afternoon.
The NZXs website also appeared to experience significant issues, with reports of similar troubles in Australia.
But most if not all services were starting to return to normal after 5pm.
ASB acknowledged a problem in a tweet shortly after 5pm, advising that its FastNet Classic and ASB Mobile services were temporarily unavailable.
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But it later said all its services had been restored.
The exchange and the banks have been contacted for comment.
The issue appeared to have also affected banks in Australia.
Australias Nine news channel reported customers were experiencing difficulties logging on to the websites and apps of Westpac, St George, ANZ and the Commonwealth Bank.
It also reported internet problems at Virgin Australia, Allianz Insurance and CMC Markets.
Online service DownDetector showed hundreds of people reporting difficulties accessing ASB and Westpacs sites in New Zealand after 4pm, and a lesser number of reports of issues with ANZ, before the volume of reports began to drop off at about 5pm.