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%.

And it's $700bn now, which is almost as much as the entire rest of the world spends on defense.

GDP is not what that defense budget is protecting. People and property are what that budget exists to protect.

China has the second highest defense budget. We have about 1/4 the population of China, yet we spend several times more on defense.