Telstra’s fixed-line and mobile internet service failed, knocking an estimated three million users offline for about an hour.

Customers could not access the Next G mobile service, fixed-line broadband, BigPond mail service and could not connect to international sites.

The carrier lost connectivity in every state at 1.50pm on Thursday, but most customers were reconnected by 3pm, Telstra’s Karina Keisler said.

“The cause of the outage is unclear at this stage, and while it’s since been restored, we’re working quickly to try to identify the cause,” she said.

Internet provider Dodo said it played a role in the failure.

Dodo CEO Larry Kestelman said one of the company’s network routers had caused an issue between Dodo and Telstra, which had “larger ramifications than it should have with other providers”.

“Even though everything is up and running now, the two companies are still investigating the root cause of the problem,” he told technology blog Gizmodo.

Microblogging site Twitter was inundated with posts from users venting their confusion and frustration, with some saying their connections were still down after 4.30pm.

Telstra was also flooded with calls to its customer service centres, creating a backlog.

Telstra would have a clearer idea of what went wrong by Friday, Ms Keisler said.

Telstra posted the following message on its Twitter page about 4.45pm: “Majority of customers are back online, however we are still working on the remaining down services.”