What areas of 'growth' are measured by the Government? Why?

Some areas of growth measured by the government are population growth, economic growth and infrastructural growth. These are measured so the Government can regulate these things, accomidate for the rising population, regulate economic growth etc.

How is this data collected? From what sources?

This is collected via the Census (population survey) and the Australian Bureau of Statistics, regular checks on economy by the government (checking employment, stock market etc).

Why do growth rates increase during period of economic stability and high employment?

Economic growth rates increase during these periods because high employment will mean that people have money to spend. This means that they will spend money, and some of this will go towards economic growth. Population may grow because other countries may be struggling, and so people come to Australia to earn money to provide for themselves and their families. Economic growth may be able to fund Infrastructural growth.