Shortly after the victory at Bukoba, the 25th Battalion moved to Voi where a military railway was being constructed for an allied advance into German East Africa. The battalion was employed in protection duties during this time, while two mounted infantry companies were formed in August by drawing upon the unit and the 2nd Battalion, Loyal North Lancashire Regiment to patrol the frontier around Maktau, 40 kilometres (25 mi) to Voi's east. Dartnell was assigned the command of a section within one of the companies.
On 3 September, word came through that a German raiding party was in the vicinity of Maktau. A counter force of 117 men was raised from Dartnell's company with Captain John Woodruffe in command, and sent to intercept the German troops. The British force, however, was ambushed by a German party approximately 200 strong, which included a number of native Askari soldiers. Casualties were high, and the situation soon descended into chaos. Woodruffe, by this time badly wounded and concerned that his men were becoming surrounded, ordered that the wounded be quickly withdrawn so that the remainder of the force could retire. However, given the intensity of the firefight and the limited resources available, the grievously wounded could not be moved. Dartnell had been wounded in the leg and was being carried away to safety when he realised the situation. Fearing what the Askari would do to the British wounded, he demanded to remain and fight. Though twice instructed to leave, Dartnell refused and gave the order for his men to abandon him. Dartnell was last seen firing on the German troops, who were within 25 metres (27 yd) of his position. When his body was recovered several hours later, it was surrounded by seven dead German soldiers.