There may have been more than one Medical Practioner by the name of Nichol in the Hunter Valley in the 1840's:
Appin G. Nicol was listed in the Government Gazette as a qualified Medical Practitioner at Cassilis in 1844 He was listed as a Medical Witness at Cassilis in 1845 
Appin G. Nicol opened a medical practice next door to Mr. Ward's Inn in Muswellbrook in July 1844 
He entered into a business in Muswellbrook called the Muswellbrook Manufactory in July 1846 although he intended to continue to practice in his medical profession, servicing both Muswellbrook and Merton.  By September 1846 he was posting notices in the Maitland Mercury stating that he was no longer connected with the Manufactory. 
Appin G. Nicol of Cassilis submitted Testimonials of Qualification to the NSW Medical Board in 1850
Appin G. Nicol was listed as a Qualified Medical Practitioner for Cassilis in 1852
1). In September 1847 the Maitland Mercury recorded a Dr Alfred John Nichol as a witness in the murder trial of John Purcell who was accused of the murder of Joseph Palfrey who had died near Cassilis. Nichol was called on to examine the body of Joseph Palfrey who had been found buried in a shallow grave. Later, he was questioned extensively at the trial as to the injuries sustained by Palfrey before death, however his findings were contradicted by Dr. McCartney who was a witness for the defence. Dr. Nichol was vindicated when, supported by his evidence, the jury found Purcell guilty of the murder of Palfrey.
2). William Nicol, Esq., was appointed Coroner for the district of Cassilis in 1848. 
3). The Maitland Mercury reported that Dr. Nicol (no first name) died at his residence at Dalkeith (Cassilis) on 15th January 1848 at the age of thirty. Thomas Augustus Perry of Merriwa was appointed Coroner for Cassilis in his place.