Australia's largest private health insurer Medibank Private has apologised to millions of its customers, after a major upgrade of its computer systems went wrong.

Key points: Problems with $150 million technology systems upgrade behind issue

Problems with $150 million technology systems upgrade behind issue CEO says he does not know when it will be fixed

CEO says he does not know when it will be fixed The ATO deadline is July 15th, 1 week from now

Australia's largest private health insurer said due to problems with its $150 million technology systems upgrade, the company would not meet next week's deadline of getting annual tax statements to customers.

The delay means millions of people will not be able to complete their tax returns, until Medibank fixes the problem.

Medibank's CEO Craig Drummond told The World Today he does not know when the problem will be fixed.

"We were aware of this issue earlier this week and we were reasonably optimistic last night that we had a solution to an identified problem," he said.

"Unfortunately that solution didn't work through the course of the night.

"Even though it's still possible that we will have member's statements out by July 15, we wanted to be very transparent and front foot with our members, with our regulators, with the media and with our shareholders and say that we have got an issue, we're working on it absolutely around the clock.

"We appreciate that it's unacceptable for our members, but we wanted to be transparent."

Medibank message to its members online.

Mr Drummond said that although the company still had eight days to solve the issue, at this stage it was possible that their customers would have to wait to file their tax returns.

"We are working around the clock to solve this issue and it's still possible that we may in fact meet that time," he said.

"But we're not giving any timeframe today because we just, the team is working absolutely around the clock to solve the problem rather than focusing on making promises that we may or may not be able to keep."

Mr Drummond has promised a full review of the computer system problems and said there would be accountability.

"I can give you an absolute categorical guarantee that there will be a full and thorough review of why we've had to go down this path today," he said.

Mr Drummond said he believed they were aware of "many of the issues" they were dealing with.

He said the reason behind not announcing a definite timeline on when the problem would be fixed was because they were not "100 per cent certain" about that date.

But he said it was highly improbable that the fix could be months away.

"But I'm not going to get cornered into giving a strong commitment on exactly what day or when exactly this is going to be resolved, because at this point in time we need a little bit more information."