Twice as good? It can be 3x or more. A senior at a top global company in Sydney can make AUD$200k (USD$154k), where a senior in Silicon Valley can make USD$350k+. Additionally, the US has levels above senior that Australia simply doesn't have, or has very little of.

Silicon Valley pays USD$500k to staff engineers. CTOs for relatively large companies in Australia don't make that kind of money.