Prime Minister Tony Abbott addressed the almost 2,000 people who had gathered at Melbourne Cathedral.
He said: "There will be a time to judge the guilty but today we honour the dead and we grieve with the living.
"We cannot bring them back but we will bring them home as far as we humanly can and we do rededicate ourselves today to supporting the bereaved to obtaining justice for the dead and for their families and to working for a better world."
Two hundred and ninety eight people died on MH17 when it was brought down over eastern Ukraine on 17 July. Thirty eight were from Australia.
Flags will also fly at half-mast across Australia to mark the day.
Catholic Archbishop of Melbourne, Denis Hart also spoke at the service which was attended by leaders of various different faith groups.
He said: "Our hearts go out to the family members of those who have died.
"We assure them of our love and our support at this special moment."
Pro-Russian separatists have been accused of attacking the jet although they deny any involvement.