Telstra and Optus have dismissed calls to delay the 3G mobile network shutdown after an inquiry warned threats to public safety were "too great to ignore".
Telstra is due to close the ageing network on August 31, having delayed its June deadline, while Optus will shut down from September.
TPG/Vodafone closed its operation in January.
A Senate inquiry examining the potential risks of the closure has been told critical medical equipment, fire alarms, EFTPOS machines and waste and water infrastructure could be cut off in the shutdown.
Users of cardiac monitors and medical alarms could be unaware their devices rely on 3G until after the network ceases operation, according to industry experts.
There have been long-running concerns about some mobile phones that will not be able to connect to triple zero on the 4G network.
The inquiry on Thursday released an interim report recommending the federal government meet urgently with Optus and Telstra to extend their shutdown deadlines to allow for an extensive audit of affected devices.
"The committee believes the dangers to public safety, business continuity and the provision of essential services as a result of the shutdown are too great to ignore and that the shutdown must be delayed until these concerns are sufficiently addressed," the report said.
Communications Minister Michelle Rowland said the telcos had made significant efforts to reach affected customers.
But she acknowledged there was more work to do, with about 77,000 mobile phones in use that will not be able to make emergency calls after the closure.
These devices - often bought overseas or second-hand - use 4G data for regular calls and texts but bump triple-zero calls to 3G because they are not enabled with a technology called Voice over LTE.
Users might not realise their phone is configured this way by the manufacturer until the 3G network is switched off.
All telcos have a service for customers to check the status of their device by texting "3" to the number 3498.
"We strongly encourage all Australians to check their device via the SMS service, respond to messages from your mobile provider and take action," Ms Rowland said.
The number of these devices in use has reduced from 740,000 in March after the government formed an industry working group.
Optus executive Andrew Sheridan acknowledged the inquiry's recommendations but said the telco was working to mitigate public safety risks.
It has sent 3.2 million messages to affected customers, provided free and subsidised handsets and engaged with businesses that manage devices such as alarms, Mr Sheridan said.
"Optus will continue to provide high levels of support to our customers during the 3G switch off through our dedicated teams and keeping device offers open."
A Telstra spokesman said the company had been warning of the need for preparation since setting the closure date of its 3G network almost five years ago.
"The vast majority of our customers have taken the necessary action to update their devices and we thank them for that. This is reflected by the fact that our 3G network accounts for only one per cent of our total mobile network traffic," he said.
"We have a robust communication plan in place through to the closure date and have established processes to help customers after the closure who have yet to migrate."
Both telcos say switching off 3G will allow them to repurpose spectrum to provide better 4G and 5G coverage, including in regional and rural areas.
They have committed to providing equivalent coverage after the closure.
