They were ordered to deal with traffic growth over the heavy gradients of the North Island Main Trunk (NIMT) and to do away with the use of banking engines on steep grades.

Further, the locomotives when hauling a full load, generated such intense heat in restricted tunnels, which are common in New Zealand, that crews disliked working them.

Also, the coal bunker carried insufficient fuel in-service and this problem was never remedied because it would have increased the axle loads beyond the light track capabilities.

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.

[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 northbound SIMT express trains faced a largely uphill fast run over the 160 km 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.

[18] Tests proved the G and KB 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 KA class locomotive rather 0.75 plates, and the link guidances were fragile, thin and insufficient in number.

The KA 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 AB tenders were fitted with only 9.75-ton axle loads.

The tenders were completely inadequate for water and coal requirements for fast 160 km, NZR runs leading to time losing extra refuelling stops.

[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.

[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 AB 663.