[4] Lavarack's division spent a month in France during September 1915, but was transferred to Salonica in Greece, where it fought in the Balkans Campaign.

He was subsequently assigned as brigade major for the 5th Division, commanding two field artillery batteries during fighting at the Somme and the advance on the Hindenburg Line.

In May 1917, his staff college training saw him transferred to 1st Division headquarters, which instigated a lifelong mutual antagonism between Lavarack and his superior at HQ, Thomas Blamey.

[2][5] By December 1917, Lavarack was a lieutenant colonel and general staff officer, 1st grade, in the Australian 4th Division, and took part in battles at Dernancourt, Villers-Bretonneux, Hamel and Amiens, with Lavarack's hand in planning for the Battle of Hamel setting the stage for several subsequent Australian victories.

[6] Back in Australia in 1929, he found himself in heavy debate with fellow IDC student Frederick Shedden over the Australian government's adoption of the "Singapore strategy".

[2][7] In 1928, Lavarack was ordered to start making plans for a possible war with Japan, which he began with a paper questioning the Singapore Strategy and argued that the Australian Army needed "mobile land forces" to counter a Japanese invasion or raids.

[8] In a paper written in 1929, Lavarack predicated that Japan would only attack the British empire if Britain were involved in a major European war, which would put into question the entire assumptions behind the Singapore strategy, namely the British would be able to move the main battle fleet to Singapore in the event of a Japanese threat.

[6] As CGS, Lavarack pulled no punches over what he saw as an over-reliance on the Royal Navy and neglect of Australia's land forces—renewing his argument with Shedden, and causing considerable friction with the Australian government, in particular a number of successive ministers for Defence.

Australia's defence strategy was based around the Singapore Strategy, where it was envisioned in the threat of trouble from Japan that Britain would send the majority of the British battle fleet to Singapore, the main British naval base in Asia, to block any Japanese advance into southeast Asia.

Lavarack had serious doubts about how practical it would be to activate the Singapore strategy, and wrote in 1935 that his motto was "trust in the navy, but keep your powder dry".

[10] Much to the displeasure of several Defence ministers, Lavarack argued that the Australian Army needed a mobile force to defend Australia against a Japanese invasion, an issue which was overruled on.

[11] At most, it was felt that the Japanese were only capable of naval raids, and the Imperial Japanese Navy would never send capital ships so far south as long as Singapore held out..[11] By contrast, Lavarack argued that the Australian Army needed more officers trained in staff work; more equipment and that the militia needed more training as he thought that the assumptions behind the Singapore strategy were flawed.

[12] At another meeting of the Council of Defence in March 1939, Blamey and Lavarack clashed again about whatever Japan would make raids upon the Australian coast or launch an invasion along with debates just how ready the Militia was for war.

[19] On 8 April, Wavell and Lavarack visited Tobruk to meet General Leslie Morshead, the GOC of the 9th Australian Division.

The lead element of the corps, a force of two battalions, reached Batavia, the capital of the Dutch East Indies, on 16 February 1942.

[32] Lavarack was recalled to Australia, where he was made acting commander-in-chief of Australian forces whilst waiting for Blamey to return from the Middle East to fill the role.

He returned to Australia in August 1946, and frustrated by his lack of active command and constantly being passed over by Blamey and others, he retired from the military in September that year.

Hanlon then offered the post to Lavarack, who accepted and was sworn in on 1 October—the second Australian-born person to hold a governorship in Australia (Sir John Northcott had been made Governor of New South Wales two months previously).

In February 1952, he proclaimed Queen Elizabeth II as the monarch in Queensland, following the death of her father King George VI.