The system, which sends SMS alerts and dials fixed lines during disasters, failed to reach large numbers of residents to warn of the toxic fire in September last year.

The slow nature of the system meant it would have required up to seven hours to send emergency alerts out to inner-north residents.

Invalid phone numbers clogged up the delivery of alerts substantially, with 22,598 calls made to Mitchell despite it having just 1500 addresses.

But Attorney-General Simon Corbell said the emergency alert system, built by Telstra and used in every Australian jurisdiction, has been fixed after the ACT aired concerns about its flaws.

Mr Corbell said a new filter will now screen out invalid telephone numbers, making the alert system vastly more efficient.

But he said it could still face delays if alerts were sent to large sections of the population. ''There's no doubt that the more numbers the system has to call, the longer it's going to take,'' Mr Corbell said.

''It's important in using the system that, wherever feasible, discreet and targeted areas are allocated for emergency alert campaigns,'' he said. ''The filter will assist considerably, I'm advised, in terms of timeliness.''

Significant problems were also caused by the system's use of billing addresses to detect those living in the inner north.

The reliance on billing addresses meant messages were either not sent to those who recently moved to the suburb, but were inadvertently sent to those who moved out of the area without updating their addresses.

Federal Attorney-General Nicola Roxon announced yesterday that a $60 million upgrade to the system had fixed those issues. Telstra phone numbers will now receive warnings based on their actual location, rather than billing address.

The fix will not be activated for Vodafone and Optus numbers until the end of next year.

''This is an important enhancement,'' Ms Roxon said.

''It cost nearly $60 million for the Commonwealth government, but we think it's a very important way of making sure those early alerts get to people quickly and get to them when they are in risk,'' she said.

The upgrades come as the ACT heads into the most dangerous period of the bushfire season and the 10th anniversary of the 2003 Canberra bushfires.

But Mr Corbell warned the emergency alerts should not be relied upon as a sole means of emergency communication.

He said Canberrans should monitor local broadcast media, online and social media, and the Emergency Services Agency website.

''People should not view emergency alert as the one and only means of receiving emergency information or to be warned about a potential emergency affecting them,'' Mr Corbell said.

''This is a very important tool but it's only one of many tools.''