One expert is warning people to do their research ahead of a significant change set to impact older devices.

One NZ, Spark and 2degrees are all due to shut down their 3G mobile networks by late 2025 - early 2026 at the latest.

Associate Professor of Engineering at the Royal Melbourne Institute of Technology Mark Gregory says Australia did the same thing at the end of last year - and it sparked mass confusion.

"In quite a few areas, customers lost coverage. So we had pictures of farmers standing in the middle of their farm where they used to have coverage...they've got 3G devices including farming sensors, farm equipment and their own devices."

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