The coordination of dynamic task allocation based on available resources is very challenging in disaster domains under time, space and communication constraints. In addition, it is also very hard or even impossible to achieve tasks allocation by using a centralised manner with the global knowledge of the working 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 range to achieve effective 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 the disaster domain. The experiment results show that the proposed approach outperforms other decentralised approaches, in disaster domains under space, time and communication constrains.