The Reserve Bank of Australia has sounded a note of caution over banks' commercial property lending, saying in its latest Financial Stability Review that this has been the main source of banks' recent business lending troubles.And it has specifically warned that Melbourne's high-rise apartments may be being propped up by foreign buyers.The RBA says its industry liaison program suggests there has been considerable building activity in Melbourne high-rises. "While these projects typically meet appropriate lending standards with strong pre-sales and sufficient equity," it says, "some rely heavily on overseas purchasers, a source of demand that might not necessarily be available in the longer term."Around 5.7 per cent of domestic commercial property loans are currently classed as impaired, up 40 basis points over the June quarter. Impairments reached 6.2 per cent at their September 2010 peak, but the RBA notes that "much of the recent improvement has been due to banks selling a few large bad debts". This may include the restructuring of the Centro property group.The Review also notes that tight lending has helped keep construction activity weak. It says the larger developers have good access to wholesale debt markets. But "industry liaison indicates that access to intermediated finance for small- to medium-sized developers is still quite tight, with lenders requiring stricter collateral and covenant conditions, and higher pre-commitment/pre-sale ratios."