Mr Swan said it was routine for the Foreign Investment Review Board (FIRB) to advise the applicant of any national interest concerns before a final decision was made.
In October 2010, the government-owned SGX announced an $8.4 billion takeover of the ASX.
"FIRB informed SGX that I had serious concerns about the proposal and that, subject to further consideration, I intended to accept the unanimous FIRB advice that the takeover would not be in the national interest," Mr Swan said in a statement on Tuesday.
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"It's important to note I have not made a final decision, and it would not be appropriate for me to make further public comments on an application that is still under consideration."
