SYDNEY (Reuters) - Australian telecoms provider Optus said on Monday that a massive outage which effectively cut off 40% of the country's population and triggered a political firestorm was caused by "changes to routing information" after a "routine software upgrade".

More than 10 million Australians were hit by the 12-hour network blackout at the Singapore Telecommunications-owned telco on Nov. 8, triggering fury and frustration among customers and raising wider concerns about the telecommunications infrastructure.

Optus said in a statement that an initial investigation found the company's network was affected by "changes to routing information from an international peering network" early that morning, "following a routine software upgrade".

"These routing information changes propagated through multiple layers in our network and exceeded preset safety levels on key routers which could not handle these," the company said.

"This resulted in those routers disconnecting from the Optus IP Core network to protect themselves."

The project to reconnect the routers was so large that "in some cases (it) required Optus to reconnect or reboot routers physically, requiring the dispatch of people across a number of sites in Australia", it added.

The vast scale of the reconnection project meant the investigation into the cause also "took longer than we would have liked".

The company added that it had "made changes to the network to address this issue so that it cannot occur again".

(Reporting by Byron Kaye; Editing by Christopher Cushing)