Though at the end of the day is the percentage against GDP that is the 'most important' (USA
48.3% Australia 17.3%). This is the percentage of what you 'owe' versus the amount you 'earn' each year.
Though like all things 'national' the ability to pay things off is determine by the value of your currency. The main issue for the USA is what their dollar is worth - luckily for the USA unlike Australia most of the rest of the world doesn't want the US dollar to sneeze otherwise they (China) would catch a cold.
P.S. if you want to get really tricky check out the 'year' slider. For example in 2000 Australia owed $95b BUT it was 25% of gdp compared to 17% now for almost 50% more in dollar value. Could have been currency based but i think it's probably productivity based.