Low interest rates have helped keep arrears on mortgage-backed securities at low levels but there are plenty of uncertainties in the property market that could upset the mortgage market."[Prime] RMBS arrears have been at 1.13 per cent, lower than the 1.25 per cent long term average," said Alisha Treacy, structured finance ratings director at Standard & Poor's Australia.Treacy was speaking as one of an expert panel in Sydney yesterday, canvassing views on the outlook for the Australian property sector.While the panel covered plenty of territory, it was anchored by a few key topics: stress on residential mortgage backed securities rated by S&P; Australia's AAA sovereign credit rating; and a range of smaller issues that could be filed under "noise in the market".And stress was certainly a relative term. Even taking what Treacy classed as a "moderate correction" of 20 per cent across the housing market, the S&P house view was that this alone would be unlikely to cause changes in 96 per cent of the AAA tranches of mortgage securities (a testament to the "credit enhancement" of these instruments, and the fact that the average loan to value ratio of the underlying collateral was 61 per cent).S&P financial services ratings director colleague Sharad Jain said the house view was not for a sharp price correction. Also, "it would not happen in a vacuum" he noted, but would be in conjunction with declines in other macro-economic measures.Jain was keen to stress that this scenario was a long way off, however, as S&P ranks Australia's economic risks and industry risks at the lowest end of a scale covering about 90 countries.Of more concern to some of the other panellists was the rise and rise of the apartment market. Paul Mirams, a partner with KordaMentha, said apartments were being constructed at three times the historical average, a situation that is unlikely to continue."We need to ask questions, although in the GFC nothing was being built for the best part of five years. Once construction started up again, it attracted a wave of capital," Mirams said."It's partly a catch-up, and won't be sustained," he said.  He said that 15 per cent to 20 per cent of apartment buyers were from offshore, and that has soaked up the existing apartments, often bought off-the-plan.This is, however, creating "blind spots" for long term property market investors, as AMP Capital Investors senior credit analyst, Tim Jarvis, explained. Property settlement often takes place offshore, so price discovery remains a mystery, as do margins made and the level of margin compression worn by the Chinese developers building many of the new apartments in Australia. Stats were contradictory; facts were hard to reconcile. The foreign buyers don't necessarily take on mortgages, and with Australian banks no longer lending to foreign buyers, there will be no effect on RMBS, Jarvis suggested.Mirams added that foreign investors were buying property for the wrong reasons, and this was likely to end badly: "Chinese buying units to shift money out of China is not the right reason