In the beginning of February 1900 it was announced that New Zealand would send a fourth contingent, approximately three squadrons, but so great was the enthusiasm and so plentiful the supply of candidates that the contribution was increased to two full battalions of Mounted Rifles, known as the 4th and 5th New Zealand Contingents. They sailed at the end of March, and disembarked at Beira at the end of April. These two corps were, along with the 6th New South Wales Imperial Bushmen, intended to form the 2nd Brigade of the Rhodesian Field Force, which crossed Rhodesia and, under the leadership of General Carrington, entered the Transvaal from the north-west; but before Mafeking was reached the brigade was split up. The two New Zealand contingents took part in the attempt to relieve Colonel Hore at Elands River. (See Rhodesian Regiment.)
In Lord Roberts' telegram of 18th August he spoke of an engagement at Buffelshoek, in which Carrington and Lord Erroll drove back the enemy in the vicinity of Elands River on 16th August, the day on which Hore was relieved by Lord Kitchener from the south. Lord Roberts remarked, "The New Zealanders particularly distinguished themselves. Our casualties: killed, New Zealand MI, Captain Harvey and 2 men; wounded, 9 men". Captain J A Harvey was a squadron commander in the 4th contingent. Captain Fulton, Indian Staff Corps attached, and Lieutenant Collins, both of the 4th contingent, were wounded.
General Carrington left the seat of war about the end of August. After that both contingents saw much fighting under Lord Methuen, General Douglas, and other leaders in the Western Transvaal. Between 9th and 12th September both had casualties near Ottoshoop, in an engagement in which General Douglas took 40 prisoners. In Lord Roberts' telegram of 22nd October 1900, he said: "Lord Erroll occupied Buffelshoek from Ottoshoop on the 19th, without any casualties, owing to the good work done by the New Zealanders under Captain Poison of the 5th Regiment Imperial Bushmen, New Zealand Contingent". In his telegram of 12th November 1900, speaking of Douglas's march from Zeerust district to Ventersdorp, the Commander-in-Chief said: "Douglas reports that the New South Wales Imperial Bushmen and New Zealanders did excellent work on the march". After this both battalions were operating in the Western Transvaal, chiefly about Ventersdorp and west of Krugersdorp, and they often saw fighting. The 4th had several casualties about 25th December, and Lieutenant Keddle of that corps was severely wounded on 12th January 1901 at Ventersdorp. The 5th was split up. During the first half of 1901 the greater portion of the regiment did column work under Brigadier General Cunningham, and afterwards under his successor, Brigadier General Dixon. In the War Record of the 1st Battalion Derbyshire Regiment occurs this sentence: "On 16th May 1901, to our great regret, we bade good-bye to the 5th New Zealand Contingent, as fine-looking and as useful a body of men as any in the field". Major Dennison, in his 'A Fight to a Finish' mentions that a squadron of the 5th contingent was in a column based on the Kimberley-Vryburg railway, which operated in the south-west of the Transvaal and in the Orange River Colony towards the close of 1900 and in 1901. They had casualties on various occasions during the first three months of 1901. Major Dennison has nothing but praise for the New Zealanders. During part of 1900 and of 1901 Captain Poison's squadron of the 5th contingent was a component part of a 'Composite Bushmen Regiment' which operated in various parts of the Transvaal under Colonel Von Donop, Royal Artillery.
In 1901 the 4th contingent did splendid work under various leaders, but it was while under General Babington that they got, and used to the full, more than one fine opportunity. In his telegram of 18th January 1901 Lord Kitchener said: "Colonel Grey, with New Zealanders and Bushmen, vigorously attacked the enemy 8 miles west of Ventersdorp, completely routing about 800 Boers. Four dead, 2 wounded, and 1 prisoner taken, many horses riderless, some rifles, etc. Our casualties, 1 man dangerously wounded". In Lord Kitchener's despatch of 8th May 1901, dealing with events in the Western Transvaal, he said: "Advancing northward from about Hartebeestfontein, early on 23rd March, General Babington pressed back the enemy and drove the main body north; Colonel Shekleton meanwhile operating against the enemy's right flank. Following up this success with mounted troops and guns only, General Babington, on the 24th, pushed on after the Boers, whose rear-guard was overtaken and driven in at Zwartlaagte. The enemy attempted to take up a second position a few miles farther north, to cover the withdrawal of the convoy, but Lieutenant Colonel Grey's New Zealanders and Bushmen overcoming all opposition, closed rapidly in on the convoy from both flanks. The enemy then abandoned guns and waggons and fled in confusion, pursued by General Babington's troops. The captures included 140 prisoners, 2 15-pounder guns, 1 pom-pom, 6 maxims, 160 rifles, much ammunition, and many carts. Twenty-two dead and 32 wounded Boers were found on the field. Our losses were 2 killed and 7 wounded". This action was by far the most successful engagement taking place in the Western Transvaal: it had a most disheartening effect on the enemy, and it was only when they found that the district was being denuded of troops to strengthen the driving columns in other parts that the Boers again commenced to show enterprise in a piece of country which was very favourable to their methods. Nothing could have been better than the conduct of the 4th New Zealand and the 6th (New South Wales) Imperial Bushmen in the action of 23rd-24th March, and both corps gained many mentions for exceptionally fine work on the part of individuals, as will be seen from the list of mentions.
The Appendix to the despatch of 8th July 1901 shows that in May the strength of the 4th New Zealand Mounted Rifles still in South Africa and on column work was 216 officers and men, with 280 horses.
When giving evidence before the War Commission Colonel Kekewich, the defender of Kimberley, was asked his opinion of the oversea Colonials. He said: "I saw a good deal of the 6th Imperial Bushmen and the 4th and 5th New Zealanders, and these were all good fighting men who knew their job well".
At the end of May and beginning of June 1901 the 4th and 5th contingents sailed for home, and in recognition of their splendid work were allowed to take with them a captured gun and pom-pom.