The inconvenience and cost of utilising existing task assignment approaches to resolve dynamical mobile task allocation. For such new domain, we first propose a method, called dynamical-sequential task allocation and reallocation, by implementing multi-round negotiation and body expansion behaviour. Every former half time step, robots negotiate sequentially and select tasks to perform, and declare the information to other robots. When all robots have finished first time selection, then the remaining unselected robots choose the remaining unassigned tasks again sequentially at the latter half time step. We set two distance thresholds for robot decision-making to apply body expansion behaviour. The advantages of our methodology are demonstrated by comparison with existing algorithms, simulation results demonstrate that the efficiency for whole system to accomplish given tasks is improved by utilising our approach. Moreover, it is more conducive to reduce the numerous computational time and communication compared with existing investigated task assignment methods.