A "slab of moisture" will bring widespread heavy rain to coastal NSW including Sydney over the next week, with localised falls topping 200 millimetres and even minor to major flooding.

The Bureau of Meteorology said a low-pressure trough off south-east Queensland and the north-east NSW coast will start to bring rain to those regions from Wednesday and those falls should then move southwards.

More of this on the way: Sydney and much of eastern Australia may collect their heaviest falls in months if not longer. Credit:Louise Kennerley

Mike Funnell, the bureau's manager of weather services in NSW, said rainfall totals would be determined by the location of the trough, and some regions could collect 50 to 100 millimetres per day.

Wednesday was "still hit and miss" but there would be higher totals as the system moved south, he said. "We could see a moderate or even major flood potential."