The federal budget deficit stood at $10.56 billion after two months of the 2013/14 financial year, Department of Finance data shows.
The financial statement for July and August released on Friday showed the deficit was running $1.25 billion larger than had been predicted in the May budget.
Lower cash receipts being only partial offset by lower cash payments was to blame.
However, a pre-election budget update released by Treasury and Finance in August forecast a $30.1 billion deficit for 2013/14, rather than the $18 billion predicted in the May budget.
The minerals resource rent tax and the petroleum resource rent tax raised a combined $601 million over July and August.
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In the update, the MRRT was forecast to raise $850 million over the full financial year and the PRRT $2.4 billion.
