A severe weather warning has been issued for large swathes of NSW for destructive winds, heavy rain, abnormally high tides and damaging surf, with the north of the state already copping the brunt of the bad weather on Monday morning. The Bureau of Meteorology also has issued a moderate to major flood warning for the Clarence Valley, including the Orara River.

Areas surrounding Mullumbimby, in far northern NSW, were drenched in more than 400 millimetres of rain in the 24 hours to 9am, while Bellingen recorded almost 300 millimetres and Coffs Harbour about 200 millimetres in the same time.

About 2000 people had been isolated by floodwaters in northern NSW by Monday morning, including about 850 people at Ewingar, while the State Emergency Service rescued 12 people from floodwaters.

Hundreds of travellers arrived at Sydney Airport on Monday to find their flights had been cancelled due to destructive winds. Jetstar, Qantas Link and Virgin Australia cancelled flights to destinations including the Gold Coast, Brisbane, Port Macquarie and Lord Howe Island.

Weatherzone meteorologist Rob Sharpe said ex-tropical cyclone Oswald would slowly move south from Queensland on Monday, battering the mid north coast on Monday before the brunt of the storm was expected to hit Sydney and possibly the Illawarra on Monday night and into Tuesday morning.

"We're expecting rainfall totals to be between 80 and 150 millimetres in Sydney, so very heavy falls, but the heavy spout is going to be during the night when people are asleep. By the time most people are going to work it will be easing up a fair bit," he said.

"There's also the risk of wind gusts up to 100km/h overnight for the Sydney area.

"Hopefully by the time the morning commute happens we will be back to a few showers, maybe the odd heavy shower."

The low-pressure system is expected to pass out to sea close to Sydney in the early hours of Tuesday morning. The rain and wind will ease on Tuesday as the system moves further offshore and into the Tasman Sea, but dangerous surf will continue into Wednesday.

The same weather system is causing an extraordinary flood crisis in Queensland, as cities and towns across the state go under water.

Dramatic rescues were under way in Bundaberg, where the city's worst flood on record has left dozens of people trapped on rooftops. At least 1200 Bundaberg properties are already flooded, and there are fears that could reach 2000.

The communities of Gympie and Maryborough are also flooding, while a major flood crisis is developing in the Lockyer Valley, where 19 lives were lost in the state's 2011 floods.

Brisbane and Ipswich are bracing for flooding expected on Monday night, with further flood peaks expected on Tuesday and Wednesday.

So far, three flood deaths have been confirmed in Queensland.

Mr Sharpe said one place in the Gold Coast hinterland had recorded more than 500 millimetres of rain in less than a day.

The Bureau of Meteorology has issued flood warnings for the Tweed, Richmond, Wilsons, Clarence, Bellinger, Hastings and Brunswick rivers in NSW.

The severe weather warning is in place for the Northern Rivers, Northern tablelands, mid north coast, Hunter, Sydney metropolitan, Illawarra and Central Tablelands forecast districts.

By Monday morning, the State Emergency Service had received 668 requests for assistance in NSW in connection with ex-tropical cyclone Oswald.

About 2000 people were also isolated by floodwaters, including 850 people at Ewingar, 100 people at Main Arm, 480 people at Uki and 13 people at the Emu Creek sour-wheel drive park.

On the mid north coast, 400 people at Darkwood and 100 people at Buccarumbi were isolated.

NSW SES Commissioner Murray Kear urged holidaymakers to consider the severe weather when making their travel plans.

"With damaging winds and heavy rain expected in Sydney, Newcastle and the Central Coast on Monday, holidaymakers should exercise patience, drive to the conditions and never enter flood water," Mr Kear said.

"If you need emergency help in a flood or storm call the NSW SES on 132 500. If your situation is life-threatening call triple-0."

At Sydney Airport on Monday, Mary-Jane Lang, 48, was one of the hundreds of travellers whose flights had been cancelled due to destructive winds.

"We flew all the way to Ballina yesterday afternoon and were turned back to Sydney because of the weather. We were about to start descending when the plane suddenly ascended again," she said.

"The pilot tried a second time and then said 'We're going back to Sydney'. I had $20 left so I got the train back down to Mittagong to stay with a friend. Then I came back this morning and the flight has just been cancelled.

"We can't go home. My daughter starts high school tomorrow. What are we supposed to do?"

- with Rachel Olding