The two were among five Defence Force Federation Guard members who passed out during the ceremony, with observers blaming the hot conditions on the lawns of Federation Place, as the soldiers, sailors and air force members stood in their full dress uniform for the 30-minute ceremony.

Three of the five were taken to hospital. One was later released and the other two were kept for observation.

In a statement, a Defence spokesman said the force would conduct a "review of the activity" after the collapses of the Defence personnel but did not specify whether the men in hospital were from the army, navy or air force.

"The Department of Defence is aware that five members of Australia's Federation Guard collapsed at the foundation stone ceremony held in Canberra today," the spokesman said.

"Basic first aid was administered.

''The ambulance on-site took three members to Canberra Hospital, one of whom has since been released. The remaining two members are currently under observation."

The department said soldiers, sailors and airmen were trained to keep themselves hydrated while in hot conditions. "Members are instructed to be well-hydrated and alternate uniforms are worn in hot weather,'' the statement said. "The Australian Defence Force has limitations on the type and duration of work in extreme weather conditions."