A couple of months ago, I saw something about Temu paying the tariffs for all of their shipments to the US in bulk but still shipping them individually.
The basic idea was that they'd figure out the tariff on everything they shipped during a time period as if it was done in one shipment, pay that, and then do individual shipments.
I suspect that something like this will happen.
Of course, there will be auditing to ensure that companies don't pay tariffs on $10M worth of goods when the actual total is $100M, but that's doable.
Australian businesses collect a value added tax (called GST) on almost all sales (some exempt goods). The same tax is payable on sales in Australia and imports.
For big intl retailers the effect is as you describe, they collect and remit the tax, then their shipments are considered tax paid when imported otherwise they would be held at customs.
It probably wouldn't be that hard to audit just with data from the major payment processors.