Govt axes 'rock solid' commitment: Abbott Opposition Leader Tony Abbott says the federal government's ditching of a planned family benefits increase is another example of Labor rewriting a "cast-in-stone" commitment. Finance Minister Penny Wong has blamed a decline in revenue for the decision to scrap a plan to increase the rate of the Family Tax Benefit Part A. "This is a difficult decision, but a responsible decision given what's happened to revenue," she told Sky News on Tuesday. The rise worth a total of $1.8 billion would have delivered as much as $300 a year for families with one child and $600 for those with two or more children. It was meant to be paid from the government's mining tax revenue, but that income is lagging well behind expectations and is likely to miss annual targets over the forward estimates. The independent Parliamentary Budget Office estimates the 30 per cent tax on the super profits of coal and iron ore producers will raise just $800 million in 2012/13, less than half the $2 billion Treasury expected. But Senator Wong said the decision to ditch the family tax benefit rise was not a cut in family payments because no one had received the extra money, which wasn't due until 2013/14. As well, the planned increase hadn't been legislated. Mr Abbott said the "rock solid" commitment made in last year's budget had been used as stick to "beat the opposition mercilessly" over family welfare. "So it didn't survive from one budget to the next, even though it was supposed to be the absolute product of the mining boom, which no longer exists in the same form, thanks in part to the policies of this government," he told reporters in Melbourne. The family payment boost was promoted by Labor as "spreading the benefits of the mining boom." "If the benefits of the boom are lower than otherwise would have been expected through ... declining commodity prices and a stubbornly high Australian dollar, then you need to adjust," Trade Minister Craig Emerson said. With just six days to go until Treasurer Wayne Swan hands down his sixth budget on May 14, Senator Wong confirmed the revenue drop for this financial year "looks to be in the order of $17 billion," from the forecast in last year's budget. Last week, Prime Minister Julia Gillard said the revenue shortfall from the mid-year budget review forecast last October was $12 billion. Shadow treasurer Joe Hockey used Twitter to highlight Labor's "budget chaos." Budget in complete chaos!!" he tweeted. Dr Emerson said playing politics on Twitter ignored the reality of the budget dilemma, which was being driven by the impact of the high Australian dollar and lower commodity prices on corporate tax receipts. The Australian Council of Social Services (ACOSS) criticised the decision not to increase Family Tax Benefit Part A, saying the government should instead focus on measures such as the Baby Bonus and Schoolkids Bonus. ACOSS chief Cassandra Goldie said she understood current pressures on the budget, but added that the government should not back away from a promise that would "greatly assist" low-income families. "In our budget submission we've argued for an urgent restructure of family payments, which would see the money from things like the Baby Bonus and the Schoolkids Bonus go back into higher payments so we get that balance right," Dr Goldie said in a statement.