This research aims to investigate whether universities should go for greater diversity in their research or specialise and concentrate their research and try to only excel in a few fields. To answer this question, we assess the association between universities' research diversification, measured by breadth and depth and their impact and ranking. Universities' diversification is measured based on the disciplinary ratings of universities in Excellence in Research for Australia (ERA 2018). Research output and citation impact data from In Cites database were used for the analysis. Breadth is measured by the ratio of the fields in which a university is active to the total number of fields. Depth is related to the ratings a university has received for its research fields. The results show a significant positive relationship between both university diversification indicators developed in this study and most of the university citation-based performance metrics and research ranking measures. In other words, universities which are active in more disciplines and are rated highly in ERA in terms of their research activity in those disciplines are more likely to have better citation impacts and rankings. Most universities appeared to be either in the high-breadth high-depth zone (including all Group of Eight universities) or in the low-breadth low-depth zone (mainly regional universities).