Disability support agencies have raised concerns about a decision to use funds previously earmarked for the National Disability Insurance Scheme to fund drought relief, saying it adds to the uncertainty and anxiety already surrounding the scheme’s success.

The government has said the new funding promise would have no impact on the NDIS, but the National Disability Service chief executive, Chris Tanti, said hearing that funds previously set aside for the scheme had been repurposed “doesn’t instil confidence.”

Tanti said that while he did not begrudge farmers any assistance, the announcement would add to the stress already felt by people with disabilities and their families.

“I don’t believe any government will let the disability community down … but there’s a lot of anxiety about this,” he said.

The $5bn drought future fund, announced on Friday, will be funded in part by $3.9bn from the Building Australia Fund (BAF), which was listed in the 2018 budget papers for the NDIS.

The finance minister, Mathias Cormann, has since said the scheme would be fully funded out of consolidated revenue, because legislation to repeal the BAF, along with a suite of other budget measures, failed to pass the Senate.

“As such, given the measures required to transfer funds into the proposed NDIS Savings Fund Special Account have not been supported by the Senate (including because of Labor opposition), the relevant special account has not been established, nor has money been transferred into it,” Cormann said in a statement.

There is absolutely no walking away from the NDIS, not now, not ever Kirsten Deane

“Instead the government has now fully and sustainably funded the NDIS out of consolidated revenue.”

Tanti said that explanation did not explain why the BAF funding was reappropriated into the drought fund.

“If we have a surplus of funds in the budget right now to fund the NDIS, why are we dipping into the NDIS funding for the drought?” he asked.

He said the NDIS clients, who had expressed concerns about delays and the scheme being overly complicated, wanted to ensure the service would be delivered as intended to the 500,000 eligible Australians.

“And at the moment, it’s not,” Tanti said.

The West Australian senator and Greens disability spokesman, Jordon Steele-John, said funding the NDIS out of consolidated revenue did not provide long-term certainty.

“That basically means that we have to bet the future sustainability of the scheme on the continued strength of the budget bottom line and the state of our future revenue, which we all know goes up and down,” he said.

Steele-John said the Greens supported drought relief but would look carefully at the potential impact on the NDIS before deciding whether to support the use of the BAF funds.

Labor’s agriculture spokesman, Joel Fitzgibbon, expressed the same reservation, saying he would “seek clarification” on the funding.

Cormann said the NDIS was currently undersubscribed but “if demand was higher in the future, the budget would accommodate that in the same way as we do for all our demand driven programs”.

The campaign director of Every Australian Counts, Kirsten Deane, said that lower than expected take-up and lower than budgeted care packages reflected the fact that the scheme was not working as intended.

She said it required significant investment to reduce the lengthy delays and ensure people were able to get the support they required.

Deane said determining how to fund the scheme was a matter for the government and it could do so however it thought best.

“But we have a very strong message back to them – that we are going to hold them to it,” she said. “There is absolutely no walking away from the NDIS, not now, not ever.”