The solving of structured multi-step word problems has been a dominant practice in schools and is seen as a way to develop students' mathematical reasoning abilities. Unfortunately, it is limited in scope and does not provide opportunities for eliciting deeper reasoning as they are neither rich enough nor anchored in authentic data that resemble real-world scenarios. Mathematical modelling activities, on the other hand, feature meaningful real-world situations and data that lead to openness to student solution and student interaction in small group learning. Coupled with the teacher's scaffolding, students are expected to expand and extend their reasoning even more towards solving the modelling task. This paper describes a case study on fostering a group of Primary 5 students' mathematical reasoning during mathematical modelling. The findings reveal that the students displayed a rich repertoire of different forms of mathematical reasoning as they generated their models. The models are sophisticated systems that helped them make decisions.