Genuine thanks, Mick. This answered my question and the supplementary question in one go:
Based on the statistical notes to the guide the risk is based on the likelihood of contracting coronavirus over a 16 week period when it is circulating in the community at a prevalence rate of 2 active cases per 10,000 people.
They then take the likelihood that you will end up in ICU after contracting it and present avoidance as the benefit, where avoidance is calculated using a vaccine efficacy of 80 percent. UK hospitalisation, ICU admission and death data supports that 80 percent efficacy assumption.
It looks to be a general base rate that doesn't address individual mitigation strategies. Clearly, if you were adopting high levels of mitigation that would impact the "potential benefits".
It seems to me that the "medium exposure risk" is like expressing the probabilities of being attacked by a shark. But for an individual who doesn't go into the water, the probabilities are zero.
If I'm currently at zero risk of contracting C-19, getting the vaccine exposes me to a risk to which I don't need to be exposed. In this wide brown land, it is possible to be at zero risk of contracting C-19 for protracted periods.
(Of course, western Sydney isn't a 10,000 square km farm with one family living in the homestead, stocked up with food and fuel. The main problem in these Sydney suburbs is cultural. Many of the residents do not trust or do not care what 'the authorities' say.)