The Russian wheat aphid (Diuraphis noxia [Kurdjumov, Hemiptera: Aphididae], RWA) was first detected in Australia in 2016 and is threatening an annual cereal industry valued at nearly 10 billion AUD per annum. Considerable uncertainty surrounds the economic risk of D. noxia to Australian cereals, which limits cost-effective farm management decisions. Through a series of inoculated and non-inoculated field trials in 2018 and 2019 in south-eastern Australia, we generated a range of D. noxia pressure metrics under different growing conditions for barley, wheat, and durum wheat. Relative yield loss was best explained by the ‘percentage of tillers with D. noxia’ (%TwRWA) with 0.28% yield loss per percent of tillers with D. noxia, which is significantly lower than 0.46–0.48% for susceptible winter wheat varieties in dryland conditions in the United States. Highest infestation levels were typically reached around GS40–50. To develop an action threshold, we calculated the rate of increase in the %TwRWA through time at 0.021% per day per %TwRWA (with little variation across sites). This allowed prediction of the expected maximum %TwRWA based on observations post tillering (GS30) and the expected duration before GS50 is reached. For earlier growth stages (<GS30), we were unable to determine yield impact and action thresholds since cereal plants produce many new tillers between GS20 and GS30, which may compensate for feeding damage as reported in other studies. This research should improve the cost-effectiveness of management decisions involving D. noxia in Australian cereal production systems.