Optus recently improved their pre-paid roaming and it seems to best almost all of the travel sim options going around, especially for convenience and country coverage. Buying multiple packs would net you a fair chunk of data too ($40 for 20gb for instance). The only caveat would be that you need to activate it in Aus first.
$20 Data Roaming
Up to $210 credit to use on data. At $0.02/MB that gives you 10GB
Prepaid Epic Data, Prepaid Epic Value, My Prepaid Ultra, My Prepaid Ultra Plus, My Prepaid Ultimate, Prepaid Ultimate Plus, My Prepaid Long Expiry, Optus Prepaid Long Expiry.
Mainland China, Hong Kong, Indonesia, Japan, Malaysia, New Zealand, Singapore, Thailand, UK and USA.
*For the cheapest method on activating a new sim, try this post - https://www.ozbargain.com.au/comment/7969742/redir (but this sim instead https://offer.optus.com.au/shop/prepaid/sim-card/2-sim)