In service

The first rebuilt locomotive, G 96, was outshopped on 8 September 1937 and dispatched north after initial tests to Christchurch for use on the Midland line. Some minor adjustments were required although the performance of the initial rebuild was deemed satisfactory and the other five engine units were subsequently rebuilt with the last locomotive, G 100, outshopped on 4 March 1938. The rebuilt locomotives were largely used between the Arthur's Pass and Christchurch section of the Midland line on heavy coal haulage during the Second World War and the immediate post-war years. The G class worked alongside the six K^B class locomotives and were able to move tonnage which would have required fourteen J class or J^A class locomotives.^[14] Only 9 trains in each direction could be run, each way, through the steep 45 miles (72 km) section from Arthur's Pass to Springfield,^[15] six regular freight, an extra run as required freight in both directions, the West Coast express three times a week and the overnight perishables mixed train 205/220. The express and perishables were hauled by A^B class locomotives, and the remaining freight trains usually by K^B class and one or two G class, or sometimes up to three G class.^[14] While difficult engines disliked by engine crews, the G class moved huge tonnage in these hard vital years, running more mileage, at lower operating costs than the A or A^A classes and in ton-miles were outperformed only by the J, J^A, A^B and K^A.^[16]

After the introduction in 1939 of the new K^B class both the G class and the K^B class were trialled on the South Island Limited and mail express on the Main South Line, during the Second World War as far south as Timaru and sometimes Oamaru and Dunedin, once certain bridges had been strengthened to accept the 14-ton axle loading of the rebuilt G and K^B class locomotives. The G class after their first full A grade overhaul in late 1941 were prepared and tuned for the trial on the summer SIMT expresses, part of the rebuilt G class purpose being an assessment of a planned new class of larger 3-cylinder Pacifics for SIMT fast Express services. The G class were cleared to operate all traffic classes on the Christchurch-Timaru on 1 Dec 1941 section including express trains.^[17] Over this demanding summer with wartime peak traffic, increased by the start of the Pacific war on 7 December 1941, the G class were extensively employed on the SIMT and West Coast express trains, until major failure on northbound train 174, the South Island Express to Christchurch on 16 January 1942 terminated their use on passenger express service.^[18] The run of the South Island expresses between Christchurch and Timaru was fast but with only a few stops, with restarts downhill on 1/100 grades. The northbound SIMT express trains faced a largely uphill fast run over the 160km from Timaru to Christchurch with difficult, uphill 1/100 grade starts with heavy wartime loads, often 14-16 carriages out of the plains with stops at Temuka, Winchester, Orari, Hinds, Ashburton, Rakaia and Burnham.

Often running late to meet the overnight steamer express from Lyttelton to meet tighter schedules than the, the fast timetable of 2hrs 54 min (express) and 3hrs 15min (mail) timetable to Christchurch, were beyond the G class, only once did a G manage the return leg of the express run back to Christchurch without major delays or failure.^[18] Tests proved the G and K^B with their high axle loads, unique complexities as booster or three-cylinder systems more efficiently deployed moving heavy coal trains on the Midland line.^[19]

The G class could often not generate enough steam to build up speed for timekeeping and being complex and rather too light, were prone to valve and motion link failure,^[16] the cylinder blocks should have been held by 1.5-inch plates as in the K^A class locomotive rather 0.75 plates, and the link guidances were fragile, thin and insufficient in number. The K^A tenders specified by the class designer, NZR Chief Mechanical Engineer, Angus would have had the same 14-ton axle load as the G locomotive. To save 2000 pounds of weight on each G class locomotive, improved light A^B tenders were fitted with only 9.75-ton axle loads. The tenders were completely inadequate for water and coal requirements for fast 160km, NZR runs leading to time losing extra refuelling stops.

After their first 1941, A grade overhaul, the G class were only leakproof for top link express service for the first 4000 miles, and each locomotive was only used for 120 days in 1941-42 to allow their express trial to be in top link condition which was only possible for each G locomotive for approximately, 20 return runs Christchurch-Arthur's Pass or Christchurch to Timaru.^[19]

While the G class were partly redesigned with express work in mind, they were incapable of the sustained 60-65mph running on the 160km northbound Timaru-Christchurch run, recovering time on heavy fast expresses over the Canterbury plains. As a result, no G class were used on express passenger trains from 1942 onwards.^[19] The G class continued to be used on regular Christchurch-Timaru express freight and stopping freight services until 1955,^[20] and were often employed on regional stopping passenger trains, such as Christchurch - Burnham and Christchurch- Springfield trains.^[21]

Although powerful, the G class had a low adhesive factor and had issues notably with steam blows created by excessive movement of the thin plate frames.^[22] The steam leaks were of particular concern to the Engine drivers, Firemen and Cleaners' Association (EFCA), as was the lack of power-reversing gear, the latter being remediated in 1941 when Ragonnet power-reversing gear was installed. Although said to run well if kept in good repair, the G class were highly unpopular and the EFCA resolved that the class could not be used in regular service after 31 March 1956 due to visibility concerns created by the steam blows.^[6]

Withdrawal

In 1954, with the locomotives requiring substantial work, and complaints from the EFCA mounting, NZR decided to stop overhauls of the locomotives.^[23]

The decision was made to retire the now badly worn-out G-class locomotives after reaching a certain mileage. Both G 96 and G 97 were withdrawn in November 1955,^[23] as having reached their allotted mileage. The EFCA then placed a ban on operating the G-class locomotives in March 1956.^[24] As a result, the remaining four locomotives remained in service but saw little use until the end of May 1956, when they were officially withdrawn.^[24] Despite their deteriorating condition, owing to the lack of available replacement locomotives, the G class did see some further use, with the final service hauled by G 100 operating on 10 May 1956.^[25] By 31 March 1957 it was reported that the G class had been supplanted by five J^A class locomotives reallocated from Otago to make up for their withdrawal.^[25]

The locomotives were then stored on "rotten row" at Linwood locomotive depot.^[26] The G class locomotives were not scrapped straight away but remained at Linwood locomotive depot in Christchurch until the early 1960s when they were broken up for scrap.^[27] Three of their tenders were used for W^AB class tank locomotives being converted to A^B class tender locomotives, with the other three G class tenders being used for A^B class locomotives.^[28] Other components, such as the steam injectors, were also removed from the locomotives for reuse on other locomotives.^[26] Boilers from the locomotives were reused as stationary boilers for generating steam, the last being in use at Whanganui's Easttown Workshops in the 1980s.^[28] One of these boilers was acquired by Tony Batchelor and transferred to the Mainline Steam Heritage Trust's Parnell, Auckland depot as a spare for A^B 663. It was later scrapped in 2018 when the Trust relocated from the Parnell depot.^[28]