Sydney house prices are set to suffer their first fall in 18 months, new figures show, in the wake of strong action from regulators designed to curb the growth in investor loans.

The survey could offer some relief to owner-occupiers who have been locked out of the Sydney and Melbourne markets, and to the Turnbull government as it plays down housing affordability as the focus of the May budget.

CoreLogic's figures for the first 27 days of April show Sydney house prices have fallen 0.1 per cent for the first time since December 2015, while Melbourne's property prices have risen 0.5 per cent, half as much as the previous month.

The full analysis for April, which could see Sydney's monthly prices fall 0.2 per cent, is due to be released on Monday and is likely to be welcomed by the Coalition after Treasurer Scott Morrison urged regulators to clamp down on "the sharp increase in the level of investor credit" fuelling the runaway property market.