Huge swells have been reported off Queensland and authorities are warning the downgraded system is still generating dangerous conditions.

Police Commissioner Katarina Carroll said conditions in the Coral Sea were expected to worsen as tide levels surge.

"The system is generating increasing seas and swells, which will coincide with an astronomical peak in high tides over the next couple of days," Ms Carroll said on Monday.

"It is a severe weather system with abnormally high tides and also powerful and dangerous surf."

Swimmers have been told to stay out of the water and popular beaches in Queensland have been closed, but not everyone is heeding the warning.

"The number of people in the water was extraordinary with surf lifesavers constantly driving up and down the beaches to tell people to get out," Ms Carroll said.

"Immediately, the second they drive past, people get back into the water.

"Listen to authorities ... when they say the beaches are shut, they are shut.

"They're not only putting themselves in danger but other emergency services, as well as surf lifesavers."

The Bureau of Meteorology said the tropical low was gradually weakening as it tracked west from its position off the Queensland-NSW border on Monday.

A severe weather warning for damaging winds along the coast and border ranges along abnormally high tides and dangerous surf.

Bureau meteorologist Helen Reid warned the system's path was unpredictable.

"The future movement of ex-tropical cyclone Seth is uncertain," Ms Reid said on Monday.

"We are expecting it to drift slowly west and possibly to the north as well over the coming days, but it is uncertain as to whether it will even cross the Queensland coast."

NSW is also on high alert for strong winds, high tides, and coastal erosion in the Northern Rivers region and other parts of the state's Mid North Coast.

NSW SES Superintendent Mark Elm said volunteers from across the northern coastal areas were keeping a close eye on the situation.

"The NSW SES volunteers are checking equipment and readying teams for support if needed," Supt Elm said.

Australian Associated Press