The boss of Australia's second largest telecommunications company has criticised the $50 billion National Broadband Network as a "ticking time bomb", while rejecting industry calls for a write-down in favour of upgrading the network.

Singtel Optus chief executive Allen Lew said cutting wholesale prices charged by the NBN Co to telcos would not solve the major problems as "people are not complaining about prices, they're complaining about the customer experience".

Allen Lew has rejected calls for a writedown of the NBN Co, instead asking for upgrades to the network. Credit:Jamila Toderas

"We believe at Optus that the NBN could well be a time bomb but we believe as an industry we have the power to work co-operatively, collaboratively with NBN to put out that fuse," Mr Lew said.

Despite this, Australian Competition and Consumer Commission chairman Rod Sims said at a telecommunications industry summit in Sydney on Monday that the cost of basic NBN plans had become an affordability issue for those on lower incomes because the charges were more than the plans some customers were on before the rollout.