The federal government says it is “deeply concerning” that half a million Australians are potentially about to lose access to the triple-zero network.

With the nation’s 3G network in its final months of operation, the latest industry figures show that up to 530,000 Australians have with devices incompatible with the 4G network.

Telstra will switch 3G off on August 31, having recently extended its deadline. Optus will shut down from September, while TPG Telecom/Vodafone began its closure in January, in a move that has been flagged for years.

Many devices – often bought overseas or on the grey market – may use 4G data for regular calls and texts, but bump emergency calls to 3G because they are not enabled with a technology called Voice over LTE.

Users may not realise their phone is configured this way until 3G is switched off.

The telcos’ estimates on how many such devices are still being used by Australians have changed from one million in April, to 400,000 earlier this month. Then this week, they said there were up to 530,000 this week.

Communications Minister Michelle Rowland said the fluctuating figures were “deeply concerning”.

The telcos have been contacting affected customers and those users should take action, she said on Friday.

Rowland said Telstra’s decision to delay its shutdown was welcome, while an industry working group is regularly updating the government.

“Options exist under law for the government to consider regulatory intervention – including proposals for delays to planned switchovers, subject to required consultation and procedural processes,” she said.

Telstra and Optus have launched services for customers to check the status of their device by texting “3” to the number 3498.

Optus executive Harvey Wright said the company had ramped up its efforts to keep users connected, including this week’s launch of the text service.

Customers could also be assured that if 3G was the only available network, it would not be switched off until 4G became available, Wright said.

“The number of sites like that are very small and we’re confident that we will be able to maintain the coverage that we’ve got in both metro and regional throughout this process,” he said.

Announcing its delayed shutdown earlier this month, Telstra said customers yet to upgrade would soon hear a short recorded message on outgoing calls reminding them to upgrade their device.

“While there is now a little more time, please don’t delay,” Telstra’s announcement said.

“Our 3G network is closing soon, and it is important you act now.”

The 3G shutdown will also affect medical devices, farm machinery and EFTPOS terminals.

Graeme Hughes, a consumer commentator and director of Griffith University’s Co-Design Lab, said those devices were of particular concern.

“I would like to see an extension of the shutdown,” Hughes said.

“There needs to be a collective approach, with co-messaging from government and Optus and Telstra … which enables the consumer to really understand what they need to do to not be at risk.”

– AAP