Borella enlisted in the Australian Imperial Force (AIF) in Townsville, Queensland, on 15 March 1915. He had to go to some effort to do so because at the outbreak of the First World War the military authorities were not accepting volunteers from the Northern Territory. Borella accepted a job as a cook for a survey party in Tennant Creek and in January 1915 he set out for Darwin to volunteer for active service. With Charlie, an Aboriginal man, he walked and swam across flooded rivers. After borrowing a horse at Powell Creek, just north of Renner Springs, he rode to Katherine where he caught the mail coach to the railhead at Pine Creek. He sailed from Darwin to Townsville on 8 March 1915 with four other men who were among the first 15 volunteers for active service from the Northern Territory.