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The competition watchdog announced on its website that it would not be able to meet the proposed May 27 decision date.
"[The delay will] allow the proposed joint venture parties to respond to the ACCC's information requests of 11 and 13 May 2010 and to provide information in addition to that requested by the ACCC," it said.
The commission will set a new proposed date for its findings once it has received the outstanding information.
The ACCC began investigating the proposed JV in December with the proposed date for its findings originally expected in late February.
Both BHP and Rio have publicly said they remain committed to the deal in light of the federal government's resource super-profits tax, despite rumours that the proposed JV might be shelved.
European regulators have launched their own investigation into the proposed merger.
The European Commission may be the major stumbling block for the deal, as it is widely believed that the commission's objections derailed BHP's takeover bid for Rio over two years ago.
The deal is expected to result in $11 billion in synergies between the two companies through cost reductions in mining, haulage and growth opportunities.
Shares in BHP have gained 24c to $37.21 in morning trade, while Rio has shed 23c to $62.65.
