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 closure.
Learn more about the 3G closure
The inquiry held its first public hearing in Cooma, southern NSW, on Tuesday to examine the end of the ageing mobile network and its potential implications.
Telstra is due to switch off 3G on August 31, Optus will close from September, while TPG/Vodafone ended 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 reliant on information from private medical companies.
Telstra's written submission to the inquiry acknowledged the challenge of reaching people with medical alarms using 3G SIM cards, because they cannot be tracked like phones.
The telco has been working with alarm providers, including the aged care sector, to make sure they are upgraded in time.
Optus will switch off 3G in September, after Telstra closes the service on August 31. (Bianca De Marchi/AAP PHOTOS)
Australian farmers use the 3G network to operate essential technology such as water monitors, tractors and security cameras, the inquiry was told.
"There will be people that don't realise that some of their ag tech ... run on 3G until after the shut down has happened," National Farmers' Federation's rural affairs general manager Charlotte Wundersitz said.
"Through this transition period, we're really seeing the connectivity and literacy barrier play out for farmers in real time."
Community groups told the inquiry locals are concerned existing mobile black spots will only get worse and leave people unable to call triple zero.
Telstra and Optus have committed to equivalent coverage and say the closure will help expand 4G and 5G spectrums.
The building industry could also be affected because many surveying tools use 3G data, Surveyors Australia chief executive Michelle Blicavs said.
Customers can text a number to check if their phone will work with 4G. (Joel Carrett/AAP PHOTOS)
"Surveyors are the lifeblood of our economy, as no project can start or finish without them," Ms Blicavs said.
"There is much concern from our profession that from September 1 they will lose all connection and works will be significantly delayed."
The government last week said 102,000 active mobile phones were not compatible with 4G for triple-zero calls, 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.
Telstra and Optus executives are due to give evidence on Wednesday.
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