An Auckland mum and her employer have been hit with a $2,500 bill from Spark after unknowingly exceeding her monthly limit by 8 gigabytes - data that would have cost $80 had she bought it in advance.

Behrokh Clark, who was on maternity leave until January 22 this year, says she was informed by her employer of the enormous bill on her work phone upon returning to the office. Clark asked for her employer not to be named in this story.

While other recent bill shock stories involved roaming charges, Clark was in New Zealand when she accrued the high charges.

Read more: 2degrees customer shocked after getting $2600 bill while on holiday in India.

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"Initially, when I received the bill from my work I thought it was a mistake or someone had hacked into my phone," she said.

Much of the usage was accrued outside work hours, which is understood to be the reason why Clark is personally liable for the bill on the work phone.

In email correspondence shared with the Herald, Spark informed Clark the full amount of $2,466 due to the company's standard charges once a data allowance cap is exceeded.

Clark had a data allowance of 15 gigabytes, but used 23.2 gigabytes, exceeding her limit by 8.2 gigabytes (which equates to 8,200 megabytes).

The Spark customer service team explained to Clark that she was charged 30c per megabyte - leading to an overall bill of $2,466.

Depending on the resolution of the video, YouTube clips consume anywhere from 100MB to 750MB of data in an hour, which would set back the user between $30 and $225 for every 60 minutes spent watching under Spark's run-on charges.

Spark has since agreed to write-off half of the amount, but Clark says the outstanding bill of more than $1000 was still too much for her at this stage.

"This is putting lots of emotional and financial stress on me as we have been on a single income for nine months due to maternity leave," she told the Herald.

"Spark should have some safeguards in place to stop data usage until such time that the phone holder contacts Spark," Clark said.

"I feel like this is irresponsible of Spark and they should have some duty of care towards their customers."

Spark has responded, saying it does provide safeguards to consumers, including numerous online and in-app tools that allow consumers to keep track of their data usage.

In addition, the company also sends warning texts to consumers once they near their limits.

"While it's clearly really unfortunate that she finds herself in this situation, Spark did clearly communicate that she was over her data limit, so it shouldn't have been a total surprise to her," a Spark spokesperson said.

"We sent her text notifications when she reached 80 per cent and 100 per cent of her data allowance, which we can see she received."

These texts inform the consumer of the costs of the data after the limit has been reached and also offer an option for add-ons.

Customers can buy 2 gigabytes for $20, so in this scenario, an additional 8GB would have cost $80 - illustrative of the significant discrepancy between bundle and run-on charges.

Clark said that she could not recall receiving these messages, but admits she may have deleted them.