Is the process fair? No I don’t think it is fair or morally right even though it may be spelt out in their T&Cs.
I think that if you recharge early then your date of expiry should be extended by the amount that recharge would normally give you. With Optus for example if you recharge with a $30 recharge 1/2 way through your current period then the 42 days should be added to the date it would have expired on the previous payment. This could allow you to plan for periods where it might not be possible to easily recharge because of things like pay dates, anticipated incapacity, or overseas travel. I do think that “pay as you go” plans are a bit more difficult to cover this way and perhaps should be treated differently.
The variety of providers each with their own T&Cs makes it hard to pick who to use for your best outcome. Requiring that recharge payments be added to current plans rather than over writing the dates would help many users better make decisions. I do note here that the recharges are for the plan and not for “extra data” or “extra” facilities eg International call credits unless included already as part of the plan.
AutoRecharge was partially covered in another topic re the 28 or 30 /31 day months for recharges Best SIM only plan provider. ALDIMobile as an example have a similar auto recharge process like Telstra and Optus, though as long as you recharge within 24 hours after your plan expiry the unused data is rolled over. They also provide similar to many others the option to “Auto-recharge” as both @phb and @PhilT both referenced in their posts.
The benefit of auto recharge with Telstra is that the same date in any month other than February are used for auto recharges gaining a user up to three extra days in a month when the plan is recharged eg December would gain 3 days and April would gain you 2 days “free” plan usage. If not on auto recharges then they use a 28 day Month for recharge periods.
For Optus their information on AutoRecharge is that a number of days can be set to be the recharge date. If you pay for the $10 plan you would need to set the number of days to 21 (this is inclusive of the date of recharge in all cases), if you pay the $15 recharge you will need to set it for 28 days, if you pay $30 or more the number of days covered under the plan are extended eg $30 to $50 gives 42 days of plan coverage and $180 gets you 365 days of coverage. They do however limit the amount of data rollover to 50 GB for any plan.
