Prime Minister Abbott was met with booing from both attendees outside the Sydney Town Hall, as well as Labor supporters inside the 1700-strong audience.
Former Liberal prime minister John Howard was also met with boos from the crowds outside.
Paul Keating and Bob Hawke received standing ovations as they arrived, followed later by Ms Gillard, who found her seat among similar applause.
Noel Pearson, Kerry O'Brien and Cate Blanchett have joined Labor’s John Faulkner in preparation to address those gathered.
Paul Kelly will also perform "From Little Things Big Things Grow", alongside performances by the Sydney Symphony Orchestra and William Barton.
Four F/A-18 Horner fighter jets will also conduct a flyover above the CBD after the service and flags on the Sydney Harbour Bridge are being flown at half-mast.
