A graduate of the University of Melbourne, where he earned his Bachelor of Medicine (MB) degree in 1903 and his Doctor of Medicine (MD) degree in 1906, Dunhill worked as a surgeon at St Vincent's Hospital, Melbourne, from 1905 to 1914, where he pioneered a new, safer surgical treatment for exophthalmic goitre, a disease of the thyroid, an operation he conducted under local anaesthesia.

He served in Egypt and on the Western Front with the 1st General Hospital and in July 1918 was appointed consulting surgeon to the Rouen area in France.

[6] Dunhill passed the entrance examinations of the University of Melbourne in English, Geometry, Arithmetic, Greek and French, but did not study Latin, which was a prerequisite for medicine.

[4] He passed his final qualifying examination at the Victorian College of Pharmacy on 11 March 1898,[7] and was registered as a pharmacist on 11 June.

[6] Charles Martin, the professor of physiology at the University of Melbourne influenced a decision by Dunhill to pursue a career in medicine.

[4][8] He graduated from the clinical school at Melbourne Hospital with his Bachelor of Medicine (MB) degree in December 1903,[9] with three first-class honours and exhibitions in medicine and in obstetrics and gynaecology,[1][10] and was appointed house physician to Henry Carr Maudsley at the Melbourne Hospital.

[2] He became a tutor in medicine at Ormond College, and was for some years a lecturer in materia medica and an instructor in clinical surgery at the University of Melbourne.

[1] Charles Martin and Harry Allen, the professor of pathology at the University of Melbourne, were advisors to Anne Daly (Mother Mary Berchmans), the rectress of St Vincent's Hospital, Melbourne, and they persuaded her to extend an invitation to Dunhill to join her staff.

When he was a surgical resident at Melbourne Hospital, he had witnessed surgeon William Moore operate on two toxic multinodular goitre patients under chloroform general anaesthesia; both died.

[4] Toxic goitre patients frequently entered the hospital emaciated and sometimes blind, and often succumbed to cardiac arrest or hyperpyrexia.

The use of local anaesthesia removed the danger of nausea and vomiting that often accompanied the use of chloroform, and patients could drink water immediately afterwards, reducing the risk of dehydration.

[2][13] Along with Hugh Devine and Anne Daly, he was instrumental in St Vincent's becoming a clinical school in conjunction with the University of Melbourne in 1910.

In the United States he visited Howard Kelly, Harvey Cushing, George Washington Crile, William Halsted and the Mayo brothers.

He then returned to the UK, where, on 13 February 1912, he delivered a paper at the Royal Society of Medicine in London outlining his surgical treatment of exophthalmic goitre.

[4][15] When Dunhill returned to Australia later that year, he became the surgeon to in-patients at St Vincent's and the chairman of the medical staff in succession to Shields, who left for the UK.

[4] On 12 February 1914, he married a widow, Edith Florence McKellar née Affleck at Scots Church in St Kilda, Victoria, in a ceremony conducted by Alexander Yule.

[17] On 1 January 1906, Dunhill was appointed a provisional captain in the Australian Army Medical Corps (AAMC);[18] his rank was confirmed on 9 February 1907.

On 14 July 1918, he was appointed consulting surgeon to the Rouen area in France with the temporary rank of colonel.

[1][26] After the war ended, Gask was appointed the professor in charge of the surgical unit at St Bartholomew's Hospital in London, and he invited Dunhill to become his assistant director.

[4] Despite his lack of a qualification, Geoffrey Keynes, who assisted him at St Bartholomew's found that Dunhill was an expert in all fields of surgery.

[27] In London, Dunhill felt ill at ease in unfamiliar surroundings,[28] and many British physicians and surgeons were sceptical of his thyroid technique.

Mortality was indeed slightly higher in the UK than in Australia, but this was because doctors clung to the old treatments, and patients referred for surgery tended to be in more advances stages of the disease.

[4] He established a Thyroid Clinic in 1931, at New End Hospital for the treatment of patients with toxic goitre and myasthenia gravis.

He applied his mind with the same zeal to his pastimes, whether they were fishing, gardening, or the furnishing of his house with choice pieces of antique craftsmanship.