The behaviour of aggregate Australian private business investment has attracted relatively little attention in the literature over the past decade or so, probably reflecting the well-known difficulties associated with modelling it. This paper reviews the main drivers of Australian business investment through a discussion of some long- and short-run trends and estimation of error-correction models for its main components. Two innovations are introduced in the modelling approach. The first is that investment in computing equipment is excluded from the models, recognising that it cannot be treated in the same way as other types of investment, particularly in light of the dramatic falls in its relative price over recent decades. The second is that standard techniques are used to exclude influential observations when modelling the short-run variation in the data as a means of accounting for the considerable volatility in these variables. This improves the robustness of the estimation. The different types of investment – equipment, building and engineering – are found to be influenced by their own idiosyncratic factors, though for each type of investment, an inverse relationship between the investment-to-output ratio and its corresponding measure of the cost of capital is found.