Varicella Zoster Virus (VZV) firstly manifests as varicella infection early in life, and then it may reactivate in older age causing herpes zoster (HZ). It is believed that re-exposure to varicella infection boosts against reactivation (exogenous boosting dynamic) delaying or suppressing HZ disease. In this study, we examine the effect that demographic changes could have had on boosting, by reducing VZV re-exposure, and consequently on VZV epidemiological trends in Australia over the 20th century. We developed a dynamic age-structured mathematical model informed with historical Australian birth and death rates to reproduce varicella and HZ incidence over time. Furthermore we analysed how different assumptions on duration of boosting and HZ recurrence affect the HZ incidence trends. The model is calibrated against age-specific varicella seroprevalence data collected between 1997 and 1999 and varicella and HZ hospitalisation data from AIHW collected between 1993 and 2009. We show that the reduction of varicella incidence follows the overall decline in birth rate. In the period 1950-2000, this reduction in varicella circulation, together with the aging population, produced a 19.1% increase in HZ crude incidence. However, following age-standardisation, HZ trends showed an increase of 3.3% over the same period. We found that demographic changes may be partially responsible for the rise in HZ incidence observed prior to and at the beginning of the varicella immunisation program. In order to explain and project more realistic disease incidence trends, future mathematical models should account for demographic changes and present age-standardised results.