The full brunt of the two-speed economy was being felt all over the country, but the worst performing state was NSW, with six of its 21 regions (mainly in Sydney) performing badly.
A Moody's senior analyst, Arthur Karabatsos, told Fairfax that Sydney was ''surrounded by a ring of fire'' - suburbs where a high proportion of householders had fallen behind on their mortgages.
He said three interest rate rises since March last year and rising costs of living were the main causes of Sydney's mortgage stress.
The city has Australia's most expensive real estate, with a median home price of $562,000.
''Sydney is an expensive place to live,'' Mr Karabatsos said.
He said the two biggest declines in mortgage performance were in mining boom states Western Australia and Queensland.
The 6168 postcode taking in Rockingham, 30 kilometres south of Perth, has the highest delinquency rate in the country with 5.31 per cent of mortgages more than 30 days overdue - more than three times the national average of 1.67 per cent.
''What I'm saying is, if you're in mining, you're sweet, if you're not, you're screwed,'' he said.
''What is happening is that not everyone is employed by the mines. Not every labourer is getting $100,000 a year, which you are if you're in the mines.
''There is definitely a two-speed economy going on and this just bears it out.''
He said national delinquencies were at a 10-year high. ''There was a bit of a spike in '08. It came down and we're now above that spike.''
Moody's said mining accounted for 2.4 per cent of employment in Queensland and 7.1 per cent in WA.
The tourism-dependent Gold Coast region had Australia's third-worst performance, with 3.11 per cent delinquencies, and also contributed the most delinquent loans in Australia - 6.9 per cent of the total.
''These regions are heavily tourism based,'' Mr Karabatsos said.
He said the high Australian dollar was putting off international tourists and encouraging Australians to go overseas for holidays. ''They're doing the Hawaii-Vegas trip and going shopping there,'' he said.
While Melbourne's performance has deteriorated along with the rest of the country, Moody's said it was performing ''relatively well''.
What boom? Defaults rise in two-speed economy
Moody's