Telstra has admitted some rural Australians may be cut off when the 3G network closes, potentially disrupting farming families' access to health and education.
Australia's telcos are forging ahead with the closure of the ageing network from the end of the month, despite a Senate inquiry urging a delay over possible threats to public safety and business.
Telstra is due to shut the 3G network on August 31, while Optus will close from September.
TPG/Vodafone ceased 3G operations in January.
The inquiry has urged the federal government to meet with telcos and delay the closure until equivalent 4G coverage is achieved and an audit of all affected devices is complete.
The committee has been told medical equipment, alarms, EFTPOS machines, farm machinery and water infrastructure could stop working after the shutdown.
Telstra is on track to provide equivalent 4G coverage, having verified 99 per cent of its sites by the end of July, according to new documents provided to the inquiry.
But Telstra's statement said it cannot guarantee that a user who has fortuitous 3G coverage will get the same service on the 4G network.
Fortuitous connectivity is common in rural areas on the fringe of telcos' coverage maps, but it can be affected by anomalies like changes to the landscape and vegetation.
"We don't include such areas in our published coverage as the coverage is unpredictable and vulnerable to change over time ... and small signal changes that may result when our mobile sites are upgraded, optimised or maintained," Telstra's response to questions on notice said.
Families who live on remote stations use fortuitous 3G for distance education when landlines and other internet sources are down, according to a submission by the Isolated Children's Parents' Association.
"If families with fortuitous 3G coverage lose access ... these students risk losing touch with their school of distance education," the submission said.
A family at Julia Creek, in outback Queensland, does not have a landline and satellite internet drops out in hot and wet weather, the association said.
"Without their mobile service they will be without contact with the outside world for months of the year, with the entire station staff having no way to contact emergency services in case of injury or for evacuations."
Telstra's statement said it provided five years' notice of the shut down, which would not affect 97 per cent of its customers.
The telco said it is committed to supporting customers after the shutdown, with a specialist service to help rural and remote residents.
Optus's response to questions on notice from the inquiry said a small number of its 3G sites had not yet been upgraded to 4G.
The 3G network will continue to operate on 45 sites in regional areas while upgrades are carried out in the short-to-medium term, it said.
