What is the change in the revenue from Australia from 2018 to 2019?
{"": {"2019": 0, "2018": 0}, "Revenue from external customers -- Australia": {"2019": 144621, "2018": 129431}, "Revenue from external customers -- New Zealand": {"2019": 13036, "2018": 8912}, "Revenue from external customers -- Total": {"2019": 157657, "2018": 138343}}
revenue_2019 = df["Revenue from external customers -- Australia"]["2019"]
revenue_2018 = df["Revenue from external customers -- Australia"]["2018"]
answer = revenue_2019 - revenue_2018