Although differentiating between fluency, accuracy and complexity, while assessing L2 task performance, is becoming a standard practice, lexical complexity as a distinctive area has received less attention in the task-based language teaching (TBLT) literature. This study re-examines previous frameworks of lexical complexity and investigates three lexical dimensions, lexical diversity, lexical sophistication and lexical density , using a structured 2 × 2 × 2 split-plot experimental design. The participants were divided into a non-planning group and a planning group and each group was further dichotomised into two proficiency levels. Each participant was assigned one familiar and one unfamiliar oral narrative task. The results show that one’s prior knowledge about a subject is associated with higher lexical diversity and sophistication, while pre-task planning promotes lexical density. The effects of proficiency seem to be largely overridden by the effects of prior knowledge and pre-task planning and show little impact on overall lexical performance. Interestingly, lexical diversity and lexical sophistication are independent of each other, and lexical density is moderately correlated with both lexical diversity and lexical sophistication. The results are discussed with reference to the Levelt model of speech production with some pedagogical implications on content-based language instruction. The exploration of the relationships between the lexical measures reveals a need for deeper and subtler characterisation of L2 lexical complexity.