A breakdown at telecommunications giant Spark impacted data, texts, and phone calls for customers in many parts of the North Island tonight.

Spark said at 8.25pm the mobile issues were "intermittent but widespread," but it was not yet sure what caused the malfunction.

The problem was one of "degradation" rather than a full outage, Spark spokeswoman Lucy Fullarton said.

"Some people are affected but then they might receive a call or text. So it's going in and out."

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An update issued about 10.45pm said it looked like the problems with mobile issues had been resolved.

"It looks like texting, calling, and data have returned to normal after tonight's problems.

Thanks to all of you who reported the bad news ... because your feedback has helped us to get this sorted as quickly as we could."

However there still seemed to be some problems for broadband customers in the lower North Island.

"There is a problem affecting some customers around the 04 region. It appears there's been an issue with the Chorus network, and we're doing all... we can to get Spark customers back online as soon as possible."

Customers in central Auckland, Hamilton, Whangarei and Wellington were among those taking to social media to report problems making calls or getting reception on their mobile phones.

Spark said some of its customers in Rotorua were also experiencing problems with their mobile services and data services.

The company said technicians were "urgently" working on the problem, which was mostly happening in the upper North Island.

The company apologised to customers on its Facebook and Twitter pages.

"Really sorry to those of you who are affected by it, we know it's a huge hassle. Our techies are working on fixing it now, and we'll update you as soon as we have more information," Spark said.

"Thanks for the update but this is impacting business," IT professional James Gander wrote on Twitter. "Please contact me to arrange refund/credit."

"It's starting to p$%s me off, I'm pregnant and having difficulties through it and need to contact my partner when he's working and now I can't," one irate Spark customer wrote on Facebook.

Spark advised customers to check the company's Facebook page for updates.