How Australian Households became the most Indebted in the World

The article is about a study that computes consolidated household debt to GDP ratio, and compares the data across 20 countries.
Denmark had long held the position, which surprises me because I have always envisioned Denmark as being the perfect 'socialist,' economy with brilliant healthcare and education system.
The GDP to debt ratio, according to investopedia, implies the country's ability to pay back it's debt, by comparing production level with debts. (http://www.investopedia.com/terms/d/debtgdpratio.asp) However, why this particular ratio is relevant is confusing to me.
The article mentions that house prices have gone up significantly and continue to soar yet it also says that the future for new homeowners and investors is not bright, which is because of lower population growth rates and hence a situation of over supply might occur in turn hurting the prices.

Link to the article: http://www.theguardian.com/business/2016/jan/15/how-australian-households-became-the-most-indebted-in-the-world