A glitch has been discovered on Vodafone's smartphone app, which saw customers randomly logged into the accounts of strangers.
Reuben Pollock told NZME he contacted Vodafone notifying them of the issue.
"This morning [Wednesday] when I opened my MyVodafone app, I discovered I was logged into a stranger's account," he said.
"I was pretty shocked and it's worrying knowing somebody else could be logged into my account."
Vodafone told Newshub that a planned upgrade of the app had resulted in an unexpected "caching issue".
"The upgrade was rolled back within 15 minutes and the caching issue corrected, however it did mean that for a period of time a small number of users were able to see some of the information customers had entered into their app," a spokeswoman said.
"Our customers' privacy is a top priority. We're urgently assessing the number of people who were impacted and the details of that impact."