Sydney could almost double its rain so far this year in just a week, with wet conditions not unlike an east coast low developing along the NSW in coming days, meteorologists say.

The "main bulk of the rainfall" will be falling over the weekend, Jordan Notara, a duty forecaster for the Bureau of Meteorology, said. "We are expecting a low to form off the coast on Friday."

While it's more likely to be a "Tasman low" than the more damaging and drenching east coast low variety, coastal regions near the system will likely see steady rain with the potential for flooding, Mr Notara said.

"We have a perfect recipe for persistent rainfall along the eastern seaboard regardless of an east coast low occurrence," Jacob Cronje, a senior Weatherzone meteorologist, said. "It almost looks monsoonal."