Thousands of flood-stricken residents in the NSW northern rivers region have been cut out of flood relief payments, forcing the government to deny allegations the emergency funding has been allocated politically.

Following catastrophic flooding, Prime Minister Scott Morrison on Thursday announced a plan to provide two additional $1000 Commonwealth disaster payments to residents in the council districts of Richmond, Lismore and Clarence Valley.

But three other council areas in the region, all struck by the floods, were not granted the two additional payments: Tweed, Byron and Ballina shires.

Two of the three council areas to miss out on the extra funding are in the Labor-held electorate of Richmond, prompting allegations that the funding allocation was politically motivated.

One shire to miss out on the payments, Ballina, is located half in Richmond but half in the adjoining Nationals seat of Page. Thousands of homes across the majority of that town were evacuated last week amid heavy inundation.

“This is an outrage,” Richmond MP Justine Elliot said.

A map showing the division of the council areas between the electorates was published as part of a popular post on Twitter.

“That is pretty clear,” Lucy Turnbull wrote.

“And very troubling.

“The whole of the Northern Rivers has been flooded. Not just certain electorates.”

Tweet from @DieselDexter

Those in the electorates to miss the additional payments announced by Mr Morrison will still be given an initial, one-off disaster payment.

More than 2500 homes in northern NSW have been declared uninhabitable following the flooding.

But the government has left the door open to expanding the scope of the funding.

A spokeswoman for the National Recovery and Resilience Agency said the initial payments focused on the three local areas to be worst-hit by flood waters, a classification determined by factors such as population and the number of requests for the initial disaster recovery payment.

“We are actively considering further impact data as we have it to hand to determine whether we need to extend that support for the three [council areas] to others,” the spokeswoman said.

Data from insurers, state and federal governments and weather experts will be consulted, the agency said.

“There is no flood event that’s occurred in this part of Australia like this,” Mr Morrison said on Thursday. “We’re looking at other impacted areas to see how we might extend that support.”

Locals in council areas to have missed out on the additional funding will still be eligible for the initial payment of $1000 per affected adult and $400 per child.

“I don’t think for one second that there is some form of parochialism where payments are being withheld from places because of the way the electorate votes,” Deputy Prime Minister Barnaby Joyce told the ABC this week.

“I would not be surprised if there was an immediate concentration on the city of Lismore because [it’s been the …] epicentre.”