Extra State Emergency Service volunteers are on their way to the mid north coast to help respond to a severe weather event.
For the second time in less than a month the region from the Clarence river south is facing widespread flooding.
The department of Primary Industries says farmers and other residents along the coastal strip from southern Queensland to the Hunter could also be affected.
Spokesman Simon Oliver said people should be preparing now.
"It looks like it's coming in through tonight and tomorrow.
"It's quite a slow moving East Coast Low.
"The centre of it seems to be around the Grafton-Coffs Harbour area, which seemed to be the heaviest hit some three weeks ago.
"Generally 100 up to 300 mlls, patchy heavy falls in amongst that.
"The last event fell on a dry catchment and we still got flooding, so this event with a similar rainfall amount if not higher, is falling on an already saturated catchment."
The chance of flooding across the mid north coast has been upgraded.
The Weather Bureau is warning of possible moderate flooding of the Coffs Creek at Coffs Harbour and major flooding of the Bellinger river.
From tomorrow there is also likely to be moderate flooding in the Clarence and major flooding of the Nambucca, Macleay and Hastings rivers.
Coffs Harbour Bureau manager Ashley Wilson said strong winds and big seas are also forecast.
"We're keeping a close eye on this low which is near stationary at the moment but we are expecting that to move west to south west today into Friday," she said.
"We are expecting some heavy rainfall in the northern rivers and down into the Coffs Harbour area later today.
'Up to 300 millimetres over the next couple of days expected and possibly more."
© ABC 2013
17:37 EDT Much of western New South Wales has begun a heat wave, reaching at least five degrees above average for at least five days, averaging a maximum of 35 degrees or more.