An overseas phone hacker has made more than one thousand calls to the triple zero emergency call service on Saturday morning, leaving some Australians waiting up to nine hours for a call back.

Over the duration of two hours, there were three short periods where 600 calls at a time were directed to Australia’s triple zero call centres, which are run by Telstra.

The incident has sparked a government investigation and is raising questions about Telstra's handling of the situation.