The advancement of technologies has resulted in the boost of a popular chatbot software -- ChatGPT. It is ripe with potential, yet has introduced various challenges, especially in the world of education. This paper aims to explore how TEFL (Teaching English as a foreign language) students perceive the usefulness and ease of using ChatGPT in regard to academic writing and how their perceptions affect continuance intention towards its usage. The researchers present a modified model of ChatGPT usage, validating it with a sample of 334 TEFL students of two Kazakhstani universities. The constructs of the model included perceived ease of use, perceived usefulness, perceived satisfaction and continuance intention. The exploratory factor analysis revealed a strong direct relationship between the constructs. Specifically, the results suggest that students who perceive ChatGPT as more useful, easier to use, and are more satisfied with its assistance in academic writing, are likely to exhibit a greater intention to continue using it in future writing tasks. However, the descriptive analysis indicated discrepancies in the data, which were elaborated in the qualitative analysis of the answers to the survey's open-ended questions, where respondents expressed doubts about ChatGPT's reliability, concerns about plagiarism and loss of writing ability, alongside with a desire for enhanced accuracy in providing reliable information. This study can serve as an initial step toward the research of the factors influencing university students' continuance intention of using ChatGPT, particularly in the context of academic writing.