‘Australia grieves with America today,’ she said in a statement on Saturday morning.
‘Like President (Barack) Obama and his fellow Americans, our hearts too are broken.’
Twenty schoolchildren and six teachers were massacred on Friday, local time, when a young gunman walked into a quiet Connecticut primary school.
Connecticut State Police spokesman Lieutenant Paul Vance said 18 children were killed inside Sandy Hook Elementary School and that two more died in hospital.
Six adults at the school were also killed, including its principal, before the killer was shot – either by his own hand or by police.
Ms Gillard said Australians would ‘hold in our hearts’ those families that had lost loved ones.
‘We share America’s shock at this senseless and incomprehensible act of evil,’ she said.
‘As parents and grandparents, as brothers and sisters, as friends of the American people, we mourn the loss of children, aged only five to ten years, whose futures lay before them.
‘We mourn the loss of brave teachers who sought only to lead their students into that future but were brutally murdered in a place of refuge and learning.’
Local media reported that the shooter began in the kindergarten section where he killed his teacher mother and her class before moving on.
Initial media reports quoted police sources identifying the shooter as 24-year-old Ryan Lanza.
However, after initial confusion, police now believed the killer was Lanza’s brother Adam, 20, other reports said.