The horrifying discovery of the infant was made in Alexander Heights in Perth's north on Monday afternoon.
Western Australia Premier Roger Cook said the baby was believed to have been inside the drain for about a fortnight.
"The mother of that child must be incredibly distressed," he told reporters on Tuesday.
It is understood tradies found the body while cleaning the drain on Monday, in what Mr Cook described as a "horrifying scenario".
"Those workers that discovered the child's body were confronted with the most distressing of circumstances," he said.
"My heart goes out to everyone involved in this horrible, horrible series of events.
"This goes to the heart of everyone's sense of humanity, a small child whose life has passed under tragic circumstances."
WA Police confirmed the investigation into the grim discovery remained ongoing.
Anyone with information has been urged to contact police.
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