Flood warnings for parts of Sydney, Illawarra and Central Tablelands

After the weather bureau issued a severe warning for heavy rainfall over the weekend, Sydneysiders are on high alert for possible flash flooding and landslips.

A coastal trough off the NSW coast could mean some areas in the Sydney, Illawarra and Central Tablelands regions could experience a month's worth of rain in the coming days, the Bureau of Meteorology BOM said.

Weather expert Jane Golding said that these are quite dangerous systems and they've been known to cause widespread flooding in the past and that's certainly on the cards for the next few days.

It was likely that rain would intensify on Saturday and Monday, which could lead to flash flooding, river flooding and landslips, according to Ms Golding.

There are minor to moderate flood warnings in place for parts of the Hunter, Central Coast, Greater Sydney and South Coast, with major flooding possible in the Hawkesbury Nepean Valley.

The rain is possibly enough to cause major flooding there again, which will be the third or fourth time in the last two years for those communities, according to our hydrologists.

The State Emergency Service SES is increasing their communication with the addition of additional staff in its call centres.

NSW SES Commissioner Carlene York has urged residents in areas prone to getting cut off by flooding to have emergency food supply prepared, as well as a radio with batteries in case of a telecommunication outage.

I don't want people to be complacent because their house or business has not been affected in the past, but it doesn't mean that it won't be affected by these forecast rain events, Ms York said.

She urged residents to stay up to date with information on the SES website and to be aware of the warnings issued over the coming days.

If we give an evacuation order, please evacuate and evacuate quickly because we can't always have an SES vehicle or our partner agencies able to access.

Roads get cut off very quickly. It's dangerous out there in the weather and we want to make sure that you make those decisions very early.

You will place yourself at risk and you will place my volunteers and emergency services at risk as well.