It is not known how many 3G-reliant medical devices, such as personal cardiac alarms, need to be upgraded before the network closure, National Rural Health Alliance chief executive Susi Tegen said.
"The devices are particularly vulnerable because ... there is very little conversation between the telecommunications sector and the device companies," Ms Tegen told a Senate inquiry into the 3G shut down on Tuesday.
Watch this video to learn more about the 3G closure
The inquiry, being held in Cooma, southern NSW, is examining the 3G shut down and its potential risks, with Telstra and Optus executives to give evidence on Wednesday.Â
Telstra is due to switch off 3G on August 31, Optus will close from September, while TPG/Vodafone ceased its operations in January.
Ms Tegen said while the federal government was in touch with a major manufacturer of the devices, the public was largely having to rely on information from private medical companies.
"We don't really know how they've kept in touch with the patients that have those devices and how much it will cost those people to upgrade," she said.
Telstra's written submission to the inquiry acknowledged the challenge of reaching people with medical alarms using 3G SIM cards, as they cannot be tracked like phones.
Telstra plans to switch off 3G on August 31 and Optus will follow in September. (Joel Carrett/AAP PHOTOS)
The telco has been working with alarm providers, including the aged care sector, to make sure they are upgraded in time.
Snowy Monaro mayor Chris Hanna told the hearing telcos had not provided enough reassurance about equivalent or improved coverage in rural areas with already patchy connectivity.
Locals endured communications outages during the Black Summer bushfires and experience mobile black spots on long trips between towns, Mr Hanna said.
"We've got a lot of farmers out there who are on tractors and big machinery that have had accidents in the past," Mr Hanna said.
"Our biggest concern is: will they be able to call triple zero?"
Telstra and Optus have committed to equivalent coverage and say the closure will help expand 4G and 5G spectrums.
Telstra's submission said it would never be economically feasible to provide terrestrial mobile coverage everywhere and Australia's land mass and topography were major challenges.
Emerging satellite technology and the federal government's mobile black spot program were improving connectivity in remote areas, it said.
Last week, the federal government said 102,000 active mobile phones were not compatible with 4G, a sharp decrease from more than 740,000 in March.
The telcos have a service for customers to check the status of their phone by texting "3" to the number 3498.
While the Senate committee is not due to report until November it has been urged to hand down initial recommendations before the shut down.
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