More than 90 per cent of the current monthly total has fallen in the past five days.
"It is now the wettest December on record. With the whole month now at 304.6mm it beats all those records for December dating back to 1867," Weatherzone meteorologist Joel Pippard said.
"There has been 274.6mm at Grafton in the last five days which is a huge amount of rainfall. It is very unusual to get it all so close together.
"We've seen a very slow moving coastal trough which has almost stalled. There is a high pressure system off the coast of New Zealand that hasn't allowed it to move very much.