Much like its sister Commonwealth Australia, the New Zealand Army of 3rd September 1939 consisted of a small Regular Force cadre mostly manning staff, artillery cadres and specialist cadres. Most of the formations were manned by men of the New Zealand Territorial Force (NZTF). Similar to Australia, the NZTF remained unmobilized at their home stations until Japan's entry in the war on 7 December 1941. Unlike Australia, the NZTF did not get drawn into the confict in the Pacific but manned home defence formations until any potential threat to New Zealand ended in the middle of 1943. Some units of the NZTF did perform garrison duties after 1941.

The first unit of the New Zealand Army to be sent overseas was No. 1 Platoon, 'A' Company, which was a detachment of two officers and 30 other ranks gleaned from the Regular Force. This small force was sent overseas on 30 August 1939 in the HMS Leander. At the same time the Regular Force and Special Reserve was mobilized and coastal defences were manned.

New Zealand's contribution to the Commonwealth war effort began with the decision taken on 6 September 1939 to mobilize a Special Force of three battalions from each of the three military districts. This force would be recruited from volunteers and from the New Zealand Territorial Force. They were 1st (Rifle) Battalion from the Northern Military District, 2nd (Rifle) Battalion from Central Military District and 3rd (Rifle) Battalion from Southern Military District. By 3 October 1939, the force became know as the New Zealand Division and the force of three battalions became 18th, 19th and 20th Infantry Battalions of 4th New Zealand Infantry Brigade. This force became know as the 1st Echelon and the entire force to be mobilized became know as 2nd New Zealand Expeditionary Force (2nd NZEF) from 12 December 1939. The 1st Echelon assembled at points in each of the military districts and was sent overseas on 6 January 1940 and arrived in Egypt on 15 February 1940. The 2nd Echelon was assembled on 12 January 1940 after the departure of the 1st Echelon. It was sent overseas on 2 May 1940, but was diverted to the United Kingdom. It arrived there on 16 June 1940 and remained there as 2nd New Zealand Expeditionary Force (UK) until 12 January 1941. It arrived in Egypt to join the rest of the New Zealand Division on 5-8 March 1941. The 3rd Echelon assembled on 15-17 May, after the 2nd had left. It departed for Egypt on 27 August 1940 and arrived on 29 September 1940. By March 1941, the entire New Zealand Division was located in Egypt in time for it to enter into its first campaign in Greece.

Two additional units were raised as part of the New Zealand Division. In England a New Zealand Anti-Tank Battery was formed on 26 October 1939. This was later retitled 34th Anti-Tank Battery, NZA and became part of 7th Anti-Tank Regiment, NZA when it left for the Middle East in April 1940. In New Zealand, the 14th Light Anti-Aircraft Regiment, NZA (41st, 42nd, 43rd Light AA Batteries) was raised in 1941 and moved to Egypt with the 5th Reinforcements on 7 April 1941. It arrived on 13 May 1941 and joined the division on its return from Crete.

Some additional specialist units were also formed at the request of His Majesty's government for service in the Middle East. These were engineer support units and are shown below:

13th Railway Construction Company, NZE - Raised late June 1940 at Ngaruawahia. To Egypt with the 3rd Echelon.
14th Forestry Company, NZE - Raised late June 1940 at Trentham. Sent to England 27 August 1940.
15th Forestry Company, NZE - Raised late June 1940 at Trentham. Sent to England 27 August 1940.
16th Railway Operating Company, NZE - Raised late June 1940 at Ngaruawahia. To Egypt with the 3rd Echelon.
17th Railway Operating Company, NZE - Raised late June 1940 at Ngaruawahia. To Egypt with the 3rd Echelon.
18th Army Troops Company, NZE - Raised 19 June 1940 at Trentham from disbanded 8th Field Company. Sent to Fiji in August 1940 and then returned to New Zealand. It was sent to Egypt on 1 February 1941.
19th Army Troops Company, NZE - Raised 19 June 1940 at Trentham from disbanded 8th Field Company. Sent to Egypt with the 3rd Echelon and replaced 8th Field Company in the division until 15 August 1941.
21st Mechanical Equipment Company, NZE - Raised 19 June 1940 at Trentham. Sent to Egypt with the 3rd Echelon.
8th Field Company, NZE- Reformed later in 1940 and left for Egypt on 1 February 1941.
In order to accomodate the training of the 2nd NZEF in Egypt, a fixed establishment was formed at Maadi Camp that would handle the need of the New Zealand Division and related units. This later took on the form of a division by 1942.

The New Zealand government also found the need to provide for the defence of Fiji in the Pacific. 18th Army Troops Company was replaced by Force 'B' which became 8th New Zealand Infantry Brigade as part of the 2nd NZEF (Pacific). This force was raised at Ngaruawahia Camp on 20 September 1940 (20th Field Company was raised at Papakura). It arrived in Fiji on 1 November 1940. On arrival, two reinforcement companies were retitled the Reserve Battalion. This later became 34th Infantry Battalion. The brigade would later be the nucleus for 3rd New Zealand Division.

Two additional forces were raised to supplement the New Zealand Army. The first of these was raised in before the start of the war in May 1939. It was titled the National Military Reserve and was designed to supplement the NZTF. By November 1940 it consisted of four battalions and fourteen separate companies. By early 1942, it had expanded into a force of twenty-two battalions. These battalions were incorporated into existing Territorial Force infantry regiments on 2 February 1942. A separate New Zealand Home Guard was created on 2 August 1940 to "augment local defences for static defence of localities, the protection of vulnerable and key points, and to give timely notice of enemy movement to superior military organization." The army took control of the Home Guard on 1 August 1941. At peak the force had 123,000 men and 137 battalions.

The New Zealand Territorial Force was not idle during the period from 3 September 1939 until Japan's entry into the war on 7 December 1941. The first units mobilized were the 1st and 2nd Heavy Artillery Groups, 1st and 2nd Anti-Aircraft Groups, and 23rd Heavy Battery, NZA, which were all mobilized on 3 September 1939. Most of the Territorial Force was partially mobilized in the period before Japan's entry into the war.

New battalions were formed in the NZTF during this period. The following new units were formed:

12th Field Regiment, NZA - 1 November 1940?
22nd Field Company, NZE - 1 November 1940?
2nd Battalion, The Canterbury Regiment - 1 November 1940
2nd Battalion, The Auckland Regiment - 1 November 1940
3rd Battalion, The Auckland Regiment - 1 November 1940
2nd Battalion, The Wellington Regiment - 1 November 1940
2nd Battalion, The Hawke's Bay Regiment - 1 November 1940. This battalion later was renamed 1st Battalion, The Ruahine Regiment to perpetuate a regiment that was absorbed by the Hawke's Bay Regiment in the inter-war period. A new 2nd Battalion was then reraised.
1st Battalion, The Ruahine Regiment - See 2nd Battalion, The Hawke's Bay Regiment
2nd Battalion, The New Zealand Scottish Regiment - 1 November 1940
Most of the raisings and mobilizations of the New Zealand Territorial Force enabled the defended fortress areas on North and South Island to be manned.