Water injection rate allocation across different injection wells in an oil and gas field undergoing secondary production is one of the most economical ways to increase hydrocarbon production. Exploiting a process systems engineering description of a rate allocation problem via mathematical optimisation can improve operational planning. We simultaneously address a production and injection optimisation problem with an economic objective function, the net present value (NPV), subject to practical constraints ensuring operational feasibility. A case study with a constant water injection rate is compared to another in which optimal dynamic injection rates and water sharing ratio among the respective injection wells are determined. With the formulated optimisation (NLP) problem, an optimal water injection strategy that reduces field water consumption and increases profitability is obtained. MATLAB's fmincon and IPOPT optimisation solvers are employed for solution, observing superiority of IPOPT compared to fmincon.