Large hailstones, heavy rainfall, damaging winds and flash flooding may affect Gunnedah and Tamworth over the next few hours.
The warning, issued by the Bureau of Meterology (BOM), on Wednesday afternoon, also included the towns of Port Macquarie, Taree, Newcastle, Gosford, Maitland, Mudgee, Bathurst, Moree, Narrabri and Dubbo.
BOM said heavy rainfall may lead to flash flooding.
Report fallen power lines to either Ausgrid (131 388), Endeavour Energy (131 003), Essential Energy (132 080) or Evoenergy (131 093).
For emergency help in floods and storms, ring the SES on 132 500.
The next warning will be issued by BOM at 6:20 pm.