Sydney, Mar 23 (IANS): Meteorologists on Tuesday warned residents of Australia's eastern coast that the threat of flooding persists in many areas after days of torrential rain, despite a slight improvement in weather conditions in parts of New South Wales (NSW) state.

Australia's Bueau of Meteorology (BoM) said "blue skies and sunshine" were expected across Sydney and the Mid North Coast on Tuesday afternoon but flooding dangers remained, reports dpa news agency.

With some 18,000 people in NSW already evacuated, another 15,000 residents were on high alert on Tuesday as two "catastrophic" weather fronts roll across large swathes of the state, state Premier Gladys Berejiklian said in a press conference.

In remarks to the state Parliament, Berejiklian spoke of "a weather incident beyond anything we could have comprehended".

Communities north-west of Sydney were ordered to evacuate amid the unrelenting rain as a massive amount of water flowed into catchments, causing rivers to break their banks.

Major flooding was occurring along the Colo River in the Hawkesbury River region. The NSW State Emergency Service ordered about 500 people in 200 homes in the area to start evacuating on Tuesday morning.

A flood peak similar was expected along the Colo River in the course of Tuesday, the NSW State Emergency Service (SES) said, and an evacuation warning applied to residents along the river.

BoM hydrologist Victoria Dodds said that there were flood warnings and flood watches in force from the northern border with Queensland "all the way down the east coast to Victoria" with a "developing flood situation in inland New South Wales.

Dodds said that colleagues in Sydney who have been forecasting for over 20 years "have never seen anything like it".

She added that there was still a very real risk of continued flooding and other dangers associated with damaging winds, hazardous surf conditions and coastal erosion.

The highest rainfall total recorded for the current flood event, 1,083 mm was at Mount Seaview, just west of Port Macquarie on the NSW coast north of Sydney, the BoM's Agata Imielska said in the press conference.

The flooding seen in the state in recent days is the worst in decades.

Communities on the Mid-North Coast were facing the worst flooding since 1929, while Newcastle, some 140 km north of Sydney, and the Nepean Valley west of the city are experiencing the worst flooding in some 50 years.