If you read the sentence just before it, 2% of GDP is the growth target: "[...] the Defence budget growing to two per cent of GDP by 2020–21". The $36b number is the target value in 2020-2021, not right now. The current US defence budget is 3.1% of GDP.

As a percentage of total government expenditure, Australia spends 6% on defence and the US spends 16%.