Foreign buyers continue to supercharge the Sydney housing market, accounting for more than 21 per cent of demand in NSW and 15 per cent-plus nationally.

Data released as part of National Australia Bank's quarterly residential property survey shows that while house price growth has slowed somewhat nationally, Sydney continues to outperform, fuelled by foreign investment.

Foreign investment in real estate is back on the table for debate.

"What we think is that it is mainly Asian [investment] and that it is mainly Chinese," NAB chief economist Alan Oster said.

Foreign buyers' interest is centred on apartments in the $500,000 to $1 million price bracket.