Re: "After the Apple update 18.3.1 I would like to know if the hearing assistance was available in Australia yet".
Have seen one post where updating to 18.3.1 has made no difference in Australia to the availability of the AirPods Pro 2 hearing feature.
In 2024: Apple rolled out the test and feature to all those countries whose regulatory authorities approved their use. It was in use in over 100 countries last year.
Approvals from late-comer countries are responded to via Apple's OS updates. Seems those countries are then appearing on the list: AirPods Pro 2 - Hearing Health Feature Availability - Apple
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Re: "And if not why not? Is it possible to get an answer on this problem from Apple?"
We are all Apple users, like yourself, in the community, so we can not speak for Apple. They announced from the beginning that approval by the relevant regulatory authorities in a country was necessary.
It has not been very long since the TGA gave approval, and other countries are still waiting too ... As far as I know, Apple does not announce ahead of time the details of it's advance-planning for OS updates.
Seems patience is required, both with a country's regulatory authorities, and with Apple. Apple does seem to be carefully responding to the approvals when given, and releasing hearing health features along with other advance-planned update features. Although for some, it's not quickly enough, am grateful to see Apple is continuing to gradually include late-comer countries, eg: like U.K.