Sydney house prices bolted ahead 30.4 per cent over the 12 months to September in the fastest annual gain on record, but the rate of growth is slowing markedly as new listings outpace sales, according to fresh data from Domain.

During the September quarter, Sydney house prices climbed by just 4.6 per cent to a median of $1.5 million, with about half of the gains achieved during the March quarter and at significantly lower rate than the previous three months when values rose by 8.2 per cent.