I am travelling from UK to Australia and my UK cell-phone provider (giff-gaff) doesn't support roaming. I will be quarantined in a hotel for two weeks and need to have a sim-card during the quarantine. Can someone suggest a solution I can buy in the UK to use in those two weeks?
You can prepurchase SIMs for most countries including Australia and get them delivered to your home at https://datago.co/.
Full disclosure: The site is run by a friend, but I have no financial interest except as a satisfied customer.
I've successfully purchased a SIM card for Australia previously on eBay. I bought an Optus Prepaid SIM, which arrived to Czech Republic within 2 weeks of purchase in regular mail. You could probably get it even faster if you pay for premium shipping. I was able to activate and top it up with extra cash in Czech Republic and it worked immediately upon arrival to Australia.
The same strategy works for almost every country in the world and personally I'd be more comfortable knowing I have one before arriving to Australia.
Using a Pay As You Go SIM from your own country (UK), when there is a network offering a good roaming deal in the country you are visiting (Australia), is usually much easier from a logistics point of view.
Australia seems to be a good example of this, as Three offers inclusive roaming there on its UK plans (including Pay As You Go):
You can use up to 12GB of data from your UK allowance in Australia per month.
They appear to currently offer a £15 pack which includes 15GB of data (up to 12GB can be used in Australia):
The SIM itself is free, and you can also use it to make calls/texts to/from the UK.
The only downside of this approach may be that 12GB isn't enough data for you.