An overseas hacker almost brought Australia’s Triple Zero service to its knees by hitting it with hundreds of robo-calls in less than two hours.

Telecommunications company Vocus has confirmed to The Age one of its customers’ phone exchange systems was compromised on Saturday, with hackers attempting what’s known as international toll fraud.

While many of the calls were blocked, those with the prefix 000 made it through.

That meant, between 6-8am on Saturday morning, emergency call centres run by Telstra received about 1800 calls.

The calls involved a recording asking the call-taker to press 55.

Telstra said there was some impact on call response times.

The incident will be investigated as part of the government’s probe into the May 4 outage linked to a fire in regional NSW.