A mum-of-three was used to budgeting $400 a month to send to Telstra — until her most recent phone bill was asking her to cough up 1000 times that.

Karen, from Bunbury in Western Australia, was left reeling when she opened her family's bill in April and saw that she owed Telstra $465,595.23.

Speaking to A Current Affair, Karen said she wondered how her modem even handled such a high amount of downloads.

"Has it been hacked? How has it actually managed to do that much data download without bursting into flames?" Karen told the program.

But when Karen tried to solve the obvious mistake, the problem only became worse, news.com.au reported.

"You can't move past the call centre, there's no-one past that, so that's when the stress and anxiety builds up — what am I going to do?" she said.

After weeks of calls with the customer service team, Karen decided to deal with someone face-to-face and headed to her local Telstra store at Bunbury.

Robbie, a worker at the store, told her it was "absolutely a mistake" and promised to get it sorted.

Blaming a "misalignment in the system", Robbie told Karen: "As far as we're aware, we're all good and moving forward."

But a month later, Karen opened her next bill and realised her bill was even more expensive.

Telstra had added her usual $400 onto the $465,000 it was trying to charge her.

"Their billing process failed, their technology failed, their customer service definitely failed and their management have failed," Karen told A Current Affair.

Telstra executive Michael Ackland eventually became involved, apologising "unreservedly" to Karen for the mistake.

"There were two issues — there was a system glitch error that created the original problem and then human error to not pick it up as quick as we possibly could," Mr Ackland said.

"(Karen) should ignore that bill because it takes a little bit of time for the billing systems to process the removal, and we have confirmed with her in writing that all those charges are removed and she should ignore them."

Tech expert Trevor Long said for a bill to cost that much, the family would have to watch 30,000 movies in the month.

"I think if there was actually that much traffic going through your home network … I don't think the average home router or home network could cope with it," Mr Long said.