Nitrogen (N) is the most limiting nutrient in intensive pasture-based dairy systems in Australia. To-date, decisions regarding N have relied mostly on generalised rules based on average pasture responses to applied N. In this paper, a new web-based application called the ‘Dairy Nitrogen Fertiliser Advisor’ (the ‘N-Advisor’) is presented. Marginal analysis and profit-maximising principles are used to assist dairy farmers and their advisors when they are considering how much N to apply to a particular paddock for a particular grazing rotation. The tool embodies rigorously defined response functions for pasture dry matter consumption that can be expected from the range of possible applications of N. These are based on dry matter yield responses of pasture to N fertiliser derived from 65 N fertiliser experiments undertaken across Australia over the past 40 years (which equate to nearly 6,000 data sets for N fertiliser - pasture yield response). A response function for the relevant region and season is calibrated by the decision-maker to the area of pasture to which the N fertiliser is to be applied. Nitrogen fertiliser recommendations developed using the N-Advisor incorporate the marginal product derived from the calibrated response function, the costs of the fertiliser (as applied) and the value of the extra pasture consumed. The N-Advisor allows users to perform what-if analyses, such as exploring the effect on the profit maximising level of N of changing the cost of N fertiliser applied, or changing the value of the dry matter consumed. The N-Advisor also enables risk associated with production outcomes to be taken into account. The aim of the N-advisor is to provide production and profitability information that has the rigour and relevance to add value to farmer decision-making about their application of N.