But the wholly-owned subsidiary of engineering and construction group Punj Lloyd last week lodged a new offer, valuing MacMahon's construction business at $35 million.
"The proposed transaction with Leighton remains the best and only transaction practically capable of being completed," it said in a statement.
MacMahon shares were down 1.5 cents, or 4.2 per cent, at 34.5 cents at 1038 AEDT.
They rose last week when the new offer from Sembawang was lodged.
An extraordinary general meeting at which shareholders will vote on the Leighton proposal is scheduled for February 26.