Telstra will replace its century-old copper wire phone network with new technology within the next 15 years, saying the ageing lines are now at "five minutes to midnight".

Telstra executives revealed the problem at a Senate inquiry into broadband services on Wednesday - the same day the company was forced to apologise again for problems with its BigPond email service.

In an email sent to customers late on Wednesday night, Telstra said the most recent email problems had to do with balancing the load on the system, rather than its actual capacity.

"The BigPond team would like to apologise for some intermittent email problems you may have experienced this week," Telstra said.

"We quickly dedicated all available resources to the problem, working to address it by rebalancing the email load."