Australian Defence Force personnel were injured when the trucks crashed southwest of the flood-prone city of Lismore, New South Wales police said in a statement.

Police sent specialist rescue units to the scene while the state's ambulance service dispatched teams of paramedics including two helicopters, officials said.

"A total of 36 patients was the last count I had," said a spokesman for New South Wales Ambulance, adding that some of the injured were taken to four hospitals.

The spokesman said he was not authorised to give further details of the defence personnel's condition.

After the crash a large army truck lay on its side across a road surrounded by emergency services and army personnel, a photograph of the scene in the Sydney Morning Herald showed.

Some of the troops' injuries were serious, Prime Minister Anthony Albanese said in a joint statement with the defence minister.

"Right now our focus is on the welfare of those involved and their families," they said.

"Our ADF heroes were on their way to help Australians in need."

Australian troops have been deployed to help people cope with the impact of ex-tropical cyclone Alfred, which has lashed a swathe of the eastern coast with heavy rain and powerful winds.

A Defence Department spokesperson said the Australian Defence Force troops were providing "community support" to Lismore after the storm.

"The welfare of ADF members and their families is a priority and Defence's focus is on supporting those involved," the spokesperson said.

Police said a specialist unit would investigate the circumstances of the crash.