In 1841, well-known convict architect James Blackburn designed the two sandstone watch houses at the entrance to St John's Avenue. Both had an entrance porch. Blackburn's original plan for No.1 had two main rooms, one for men and one for women, and three smaller rooms (cells?). No.2 had three main rooms for constables.
Blackburn's watch houses may not have been the first on the site. Charles Bruce's etching in 1831 depicts two watch houses in a similar location but there is no evidence that they were built. In the early 1840s, it was reported that
A Watch house is very much required at Newtown on the main road - where the population is thick - and I should[?] feel obliged by [His Excellency] authorizing a sum not exceeding 200 pounds to be paid for this service - when I will arrange with the Director General of [Public Works] to put up the building - The Head master of the Queen's Orphan School is very anxious to have a Watch house and [Dl. Com] residence as lodges at the entrance to the Orphan School property heading to the Church [wh?] be equally [….?] 
The watch houses have had a variety of uses and are also know as gate houses. At one stage, No.1 St John's Avenue was used for the Hobart District Nursing Service.
The black oak trees (Quercus species) on other side of the Avenue were planted to commemorate Queen Victoria's Diamond Jubilee in 1897.
C. Bruce, Front view of the New Church and King's Male and Female Orphan Schools now in progress of building at New Town, January 1831 etched by C. Bruce.
Harley Preston, 'Blackburn, James (1803 - 1854)', Australian Dictionary of Biography, Volume 1, Melbourne University Press, 1966, pp 109-110.
TAHO, CSO 5/1/283/7452
 TAHO, CSO 5/1/283/7452