The coordination of dynamic task allocation based on available resources is a very challenging issue in disaster domains under time, space and communication constraints. In addition, it is also very hard or even impossible to achieve tasks allocation in a centralised manner with the global knowledge of such an environment. This paper presents a novel decentralised coordination approach for dynamic task allocation by considering space, time and communication constraints in a disaster domain, and workloads and priorities of different tasks. In this approach, a group formation mechanism is proposed to help agents with limited communication ranges to achieve efficient task allocation in a group through cooperation. The overall task allocation is achieved through distributed coordination in each dynamic group without a central control mechanism to reflect real life situations in a general disaster domain. The experiment results show that the proposed approach outperforms other decentralised approaches, in disaster domains under space, time and communication constrains.