"The NSW State Emergency Service is directing residents to evacuate immediately where safe transit exists and they are able to do so," the order issued about 4pm said.
"Do not delay your evacuation. Roads will be congested or closed. You could become trapped and need rescue."
The Bureau of Meteorology says major flooding is forecast along the Tweed River with levels predicted to be similar to 2001 and 2008 floods.
Evacuation centres have been established in Lismore and Kingscliff, ABC reports.
Earlier, evacuation advisories were issued to people downstream of two Queensland dams earlier today, and to people in low-lying areas of the Tweed River near Murwillumbah, as the remnants of Cyclone Debbie smash southeast Queensland and northern NSW with fierce winds and torrential rain, and rising floodwaters.
Residents outside the Queensland town of Biloela are being urged to relocate as authorities issued an emergency alert for water levels at the Callide and Kroombit Dams.
Police say residents downstream should consider leaving "in case water releases are required".
Schools were closed down in southeast Queensland today in an unprecedented move, and will remain so tomorrow.
"It will get worse"
It comes as the heaviest downpour is expected to hit Brisbane, the Sunshine Coast and the Gold Coast shortly before peak hour today, creating a nightmare commute for those returning home from work.
The head of Queensland's Fire and Emergency Services has warned nowhere in southeast Queensland is safe from the destructive aftermath of Cyclone Debbie set to lash the area this afternoon.
Speaking with ABC radio, QFES commissioner Katarina Carroll said the system was so large and widespread, no parts of Brisbane and its surrounding areas were any safer than others.
"Whether you're on the Sunshine Coast or the Gold Coast or in north Brisbane or South Brisbane, we're all going to be subjected to severe weather this afternoon," she said.
Flooding has already begun and Queensland residents are scrambling for sandbags as they prepare for the deluge no one saw coming.
Areas of the Gold Coast Hinterland have already had up to 412mm of rain in the past 24 hours with conditions to worsen.
"The situation will get worse before it gets better," Carroll warned.
