I am not sure what the definition of "mortgage stress" or "severe mortgage stress" is, but 45% of first-home buyers are now going through mortgage stress, according to Aussies Struggle to Foot Mortgage Bill.
We know that the residential real estate crash in America was triggered by subprime borrowers, that is, home buyers who purchased houses they could not afford. The article above seems to suggest that many Australian borrowers are also in the same boat. They purchased too much house and, as interest rates rise, they will struggle more and more.
One hope is that incomes rises enough so that these borrowers can continue to afford their mortgages.