Bringing forward the Coalition’s income tax cuts would give high-income earners an extra $104 a week while low- to middle-income earners would gain just 50 cents to $4 a week, according to new modelling.

The Australian Council of Social Service released the modelling on Monday to warn the Morrison government off any attempt to bring forward phases two and three of its income tax package in Tuesday’s budget.

The treasurer, Josh Frydenberg, will deliver the economic statement just days before the start of the 2019 election campaign, hoping to use a return to surplus in 2019-20 and infrastructure spending to win popularity.

Frydenberg has done nothing to hose down expectations of tax cuts for low- and middle-income earners, which would come on top of $285m in one-off payments to pensioners and other welfare recipients, except those on Newstart.

According to Acoss, if the government brings forward tax cuts due to start in 2022 and 2024 by flattening the tax scales so workers earning between $40,000 and $200,000 pay the same rate, households earning more than $200,000 would gain $104 a week.

Households with an income of between $50,000 and $75,000 a year would receive an average of $4 a week, while those earning between $25,000 and $37,000 would receive an average of 50 cents a week.

Even bringing forward just the first stage of tax cuts would see households earning $200,000 gain $42 a week, while those on less than $100,000 take just $5 a week extra.

Cutting the lowest tax rate from 19% to 17.5% would be more progressive but still offer a smaller benefit to low-income households in absolute terms since around one third of them have incomes too low to pay income tax.

The Acoss analysis concluded that increasing Newstart is the most effective way to support people on the lowest incomes, with almost all of the gains of increasing the unemployment benefit by $75 per week flowing to the lowest 10% of households by income.

The Acoss chief executive, Cassandra Goldie, said “most households in the lowest 40% by income – which includes many pensioners, people locked out of paid work and low paid workers – 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,” she said.

“It’s grossly unfair to be handing those households on more than $200,000 a year an extra $100 a week, while we have three million Australians living below the poverty line.”

In 2018 Labor proposed to re-engineer the Coalition’s $144bn seven-year income tax cut package, refocusing it on low- to middle-income earners to almost double the tax relief from 2019-20 for up to 10 million workers.

The original Coalition package will give 4.4 million taxpayers with incomes between $48,000 and $90,000 a $530 cash rebate in July.

Labor will increase the rebate to $928 next year at a cost of $5.8bn over the forward estimates, with four million people earning between $50,000 and $90,000 getting the full rebate, and 10 million getting a portion.

The 2019 budget papers roll off the presses in Canberra on Sunday in preparation for Tuesday’s budget. Photograph: Mike Bowers/The Guardian

On Sunday the shadow treasurer, Chris Bowen, braced for a Coalition income tax cut splash by arguing the government “has a lot of catching up to do” to improve on Labor’s offering. “We have 75% bigger tax cuts than the government earned for every Australian who earns less than $125,000 a year.”

On Monday the National Centre For Social And Economic Modelling released analysis showing that Labor holds eight of the 10 seats with the highest rates of poverty: Blaxland, Fowler, Watson and McMahon in New South Wales, Calwell and Bruce in Victoria, Lingiari in the Northern Territory and the notionally Labor new seat of Spence in South Australia.

Electorates with the highest rates of inequality tended to be in inner-city Melbourne, with Kooyong, Melbourne, Goldstein, Higgins, Chisholm and Macnamara – four of which are Liberal held – in the top 10.

The other electorates with the highest inequality were Lingiari and Sydney, held by Labor; and the Liberal seats of Menzies and Bradfield.