What is a good AUSTERITY indicator?
If it is about European Union, it would make sense to look at austerity as a response to the debt crisis. I would measure austerity not as the level of government debt, but as the change in levels of government debt from year to year. The slower the government debt (as a share of GDP) increases, the more austere are the budget policies of that country. 

Also, if you have access to state budget deficits data, it would be a very convincing measure of austerity in my opinion. The less is the budget deficit, the more austere is the budget policy.