Emergency assistance organisations stand to lose $5.7m a year from 2020 after the social services minister, Paul Fletcher, refused to commit to extend additional funding in the budget.

This has prompted Labor to call for funding certainty for organisations that provide one-off assistance – such as food hampers and transport vouchers – to those in financial hardship.

The Department of Social Services has disputed claims of a cut but conceded that $5.7m of additional funding is to expire in December.

On Saturday, the government is expected to unveil a fix in the disability area after the National Disability Insurance Agency board met on Friday to consider price increases demanded by service providers, which could cost more than $500 million over the next 15 months.

But questions remain about whether any potential underspend in the national disability insurance scheme will be reallocated to the budget surplus or reinvested in services.

With Tuesday’s budget expected to show a return to surplus in 2019-20, the Morrison government is keen to use the economic statement as a springboard for re-election. It is expected to announcetax cuts for low and middle-income earners and an infrastructure package to bolster its stocks in key marginal seats.

In an open letter spearheaded by the Australian Council of Social Services, the community services sector is calling on the government to increase social spending by lifting the Newstart allowance instead of pursuing further income tax cuts, as the treasurer has signalled.

Employer groups and the education sector expect the budget to have a significant skills component, pointing to Scott Morrison’s commitment in November that “every single cent” of the $1.5bn skilling Australians fund established in the 2017 budget will be spent on vocational education.

At least $160m of the fund remains unspent, in part because Victoria and Queensland have not signed up to the scheme, which requires matched funding from the states.

In pre-budget consultations, employer groups including the Australian Chamber of Commerce and Industry have suggested that money be reallocated towards incentives for apprenticeships and more support for vocational training.

On Friday, the resources minister, Matt Canavan, announced that resource and energy exports reached a record high of $278bn in 2018-19, up $50bn on the preceding year.

Deloitte Access Economics estimates the budget will be $3.1bn better off in 2018-19 and $5.7bn better off in 2019-20, compared with the official mid-year update in December, thanks to rising commodity prices and company tax receipts.

The shadow assistant minister for families and communities, Jenny McAllister, told Guardian Australia the government’s priority in the budget should be to “reverse the $5.5 million of cuts to emergency relief organisations and make sure that charities providing services have secure funding into the future”.

In November, the Coalition provided a $5.5m one-year extension of funding to a clutch of financial counselling and charity groups that provide one-off assistance such as food vouchers or parcels, partial payment of utility accounts, clothing, and budgeting assistance.

That funding is due to run out in 2019 if not extended in the budget, with organisations including the Red Cross, BaptistCare and Anglicare set to lose payments. Labor has committed to give the organisations $10m a year for four years.

“Emergency relief organisations are providing support to Australians who are falling through the cracks,” McAllister said. “These charities are finding themselves increasingly stretched and need more resources.

“The real test for the government will be whether they join Labor in supporting Australians in financial hardship by reversing their funding decision in the budget.”

The department said there had been “no cuts to emergency relief funding”, and it remained at $200m for the next four years.

It said the financial wellbeing program was “redesigned so that services continue to be appropriately targeted, better integrated and meet government policy priorities”.

Explaining the funding cliff at the year’s end, it said additional funding had been provided only “to support the establishment of the new services and to allow time to for vulnerable clients to transition to the new providers”.

In the open letter, a group of community sector organisations called on the government “not to rush to pass more income tax cuts before the elections, and to reverse those already legislated to go to high income-earners after 2020”.

The letter is signed by Acoss, the St Vincent de Paul Society, Anglicare, the Australian Unemployed Workers Union, Carers Australia, and the Council to Homeless Persons among others.

“It is unconscionable to give away more tax cuts, on top of the $227 a week already going to people on $200,000 a year, while three million people live in poverty,” it said.

“Most households in the lowest 40% by income get no benefit from tax cuts. It’s not good enough to offer temporary bonuses to people receiving social security when high income-earners get permanent tax relief.”

The group proposed lifting Newstart by $75 a week from its current level of $40 a day. The letter said the measure would cost $3bn, one quarter of the cost of the tax cuts already legislated for taxpayers on an income of more than $90,000 a year.

According to a UComms poll of 1,635 Australians conducted for the Australian Council of Trade Unions, 72.8% of respondents want the budget to contain “concrete measures to address low wages growth and excessive casualisation of jobs”.

Given a binary choice for which is a greater priority for the budget, 62.3% nominated “more investment in Medicare and government services” compared with 37.7% who asked for further income tax cuts.