"It is a very dangerous operation, especially any operation involving special forces on the ground," he said.
"They are very highly trained men, but they carry out incredibly risky, daring and dangerous tasks and in those circumstances the possibility of death, the possibility of casualties, is quite high."
The timing of the deployments has not been announced and full details will remain secret.
However, Howard indicated that some will be leaving almost immediately and most will be in the region by mid-November.
In a show of bipartisan support Howard and Opposition leader Kim Beazley will together farewell some units, and Beazley has given an undertaking to continue all military support to the campaign if he wins the November 10 election.
The Royal Australian Navy had scaled down its patrols against people smugglers in the Indian Ocean before yesterday's announcement, and the RAAF had cancelled a major air show when forces were put on standby after the September 11 terrorist attacks on America.
Under the deployment announced by Howard the guided missile frigate HMAS Sydney will replace HMAS Anzac to extend Australia's United Nations sanctions duty in the Persian Gulf, with the possibility of joining the anti-terrorism campaign later.
Two other frigates and an amphibious command ship - probably the helicopter-equipped HMAS Kanimla - will join the US-led coalition fleet.
The RAAF will send four F/A-18 fighters, two P3 Orion patrol aircraft and two Boeing B707 air-to-air refuelling tankers.
The commitment is open-ended, and although the Australian forces will remain under ultimate national command they will operate under the orders of American commanders in the region.
About 1550 Australian troops, sailors and airmen will be involved.
The decision to send forces to Afghanistan was made within days of the destruction of the World Trade Center through Howard's invocation of the mutual-defence provisions of the Anzus treaty, but the final commitment was made after a call from President George W. Bush on Tuesday night.
Howard said that, in a sombre conversation, Bush had sought Australian forces and had expressed his gratitude for the nation's readiness to join the campaign.
"This is an exercise on behalf of all the Australian people and is something that expresses the will of the Australian people, the repugnance the Australian people feel for the terrorist threat, and the determination of the Australian people to be part of the response of the free, civilised world to that threat," he said.
Beazley said the commitment of Australian forces to the campaign had the full support of Labor and of the Australian people, but warned that it would be a long, dangerous war of a kind the nation had not waged before.
"This is a different sort of conflict [which] may go on for some considerable time," he said.
"We are seeking here not the control of territory. What we are seeking here is a group of international terrorists which threaten the wellbeing of all of us."
Map: Opposing forces in the war against terror
Afghanistan facts and links
Full coverage: Terror in America
