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This paper reports on a quasi-experimental study designed to explore the relationship between lexical diversity and academic vocabulary use among Indonesian EFL university students engaging in online academic writing discussions and assignments. The study analysed 11,624 tokens and 5437 types collected from students' contributions in online discussions and academic writing assignments. Findings revealed that the level of lexical diversity among the students was significantly varied depending on the text length and the total amount of data. It was found most students were indicated to possess average ESL level of academic writing with a D value ranging from 50 to 70 while some others indicated a high level of adult ESL with developed academic text in the range between 70 and 80 D level. Moreover, it was also found that the degree of lexical diversity was not affected by the writing topic familiarity or students' academic year. Findings revealed a statistically significant negative correlation between measures of lexical diversity and academic vocabulary use. Contrary to expectations, while a diversified vocabulary generally correlates positively with student writing performance, this relationship reverses in contexts emphasizing the academic quality of writing. These results highlight the complex dynamics of lexical usage in academic writing and suggest that mere lexical diversity does not always equate to higher academic writing quality. This study contributes to a critical understanding of how lexical diversity functions in digital academic environments and offers implications for English language teaching practices in higher education settings.