On 9th January 1947 the New Zealand Permanent Force and the Territorial Force were amalgamated to form the New Zealand Army with The New Zealand Infantry Corps being established also on this date.   Over this period, New Zealand’s contribution to the Allied Occupation Force of Japan was J Force, which comprised the 9th New Zealand Infantry Brigade.  This further comprised the Divisional Cavalry Regiment, the 22nd and 27th New Zealand Infantry Battalions, which on this date were redesignated as the 1st, 2nd, and 3rd Battalions, the New Zealand Regiment (1 NZ Regt, 2 NZ Regt and 3 NZ Ret respectively with the Div Cav Regt becoming the 1st Bn). In 1948 the 9th NZ Infantry Brigade was withdrawn from Japan and the three Battalions deactivated.

Prior to this, regular infantry personnel belonged to either the New Zealand Staff Corps (as Officers) or to the New Zealand Permanent Staff (as Non-Commissioned Officers and soldiers).  The formation of The New Zealand Regiment now provided for the first time a regiment to which regular infantry officers and soldiers could belong to.

The height of the Malayan Emergency forced New Zealand to redirect its national defence strategy towards Southeast Asia, and in 1955 a New Zealand Special Air Service squadron was raised and trained for service in Malaya.  In December 1957 the squadron was replaced by The First Battalion The New Zealand Regiment which was formed in Waiouru on 1 August 1957 through the simple expedient of reactivating 1 NZ Regt.   After a period of build-up training and a parade through the streets of Wellington, the battalion boarded the SS Captain Cook and sailed for Singapore on 28 November 1957.  The battalion formed part of the 28th Commonwealth Infantry Brigade Group in the Ipoh Taiping area of Northern Malaya and commenced anti-terrorist operations in the jungles of Perak State.

In July 1959 the 2nd Battalion The New Zealand Regiment (2 NZ Regt), was reformed at Waiouru Military Camp with the purpose of relieving the 1st Battalion on completion of its two-year tour in November 1959.

With the official end to the ‘Emergency’ on 31 July 1960, the battalion reverted from a fully operational role and participated in numerous exercises with occasional short operations along the Malaya-Thailand Border. In November 1961, the 1st Battalion (on its second tour) in-turn relieved the 2nd Battalion (who returned home to Burnham) and moved into 28th Commonwealth Infantry Brigade Group's new base, Terendak Camp, in the State of Malacca. During 1963 the New Zealand Army announced its plans for the formation of a new single infantry regiment. This plan included the amalgamation of the 1st and 2nd Battalions, The New Zealand Regiment, so, accordingly the planned relief of 1st Battalion by the 2nd Battalion in November 1963 did not occur. During November 1963 the 2nd Battalion was disestablished and became the 1st Battalion Depot.

On 1st April 1964 the ten separate Regiments of the Royal New Zealand Infantry Corps became seven numbered battalions of the new single Royal New Zealand Infantry Regiment with the 1st Battalion The New Zealand Regiment became the 1st Battalion, Royal New Zealand Infantry Regiment (1 RNZIR). On the formation of 1 RNZIR it was decided to continue to observe 1st August as the Battalion's birthday as the 1st August 1957 marked the birth of the first infantry Battalion in the New Zealand Army consisting completely of regular officers and soldiers.

Because it is recruited on a nation wide basis, and had no specific regional links, the 1st Battalion, Royal New Zealand Infantry Regiment claimed descent from, as well as 1st and 2nd NZ Regts, all previous Territorial Infantry Regiments of the New Zealand Army.

Except for a six-month period in 1965, and another in 1966, when it was under the operational control of the 19th United Kingdom and the 99th Gurkha Infantry Brigade Groups respectively, the Battalion served continuously with the 28th Commonwealth Infantry Brigade in Malaysia and Singapore.

The Battalion has also had sub-units detached for service with the 1st Australian Task Force in the Republic of Vietnam - Victor Company from May 1967 to 1971 and Whisky Company from December 1967 until November 1970.

In December 1969 the Battalion moved from Terendak Camp in Malaysia to Singapore to an interim home in Punjab Square Nee Soon Garrison, and  then in June 1971 to Dieppe Barracks.   In August 1989 1 RNZIR returned back to New Zealand after 32 years service in Southeast Asia.

The Battalion marched in to its new home Wellington Lines in Linton Military Camp in August 1989. This was followed by a march through the streets of Wellington which represented a home coming for the Battalion who had marched through Wellington in 1957 en route to operations in Malaya.  Since its return, the battalion has returned to Southeast Asia on exercise on a number of occasions and has deployed personnel on various overseas operations including Bosnia (1994 - 1995), East Timor (1999 – 2002, 2006 -2007) and Afghanistan (2003 - 2007).