This paper describes a collaborative analytical technique employed by a team of subsurface, production and facilities engineering members on improving production allocation in a mature field producing at a high water cut. The field production allocation deteriorated sharply from 2015 when a field water cut was beyond 90%. The paper describes the entire workflow starting with problem identification, preliminary investigation, key actions taken, and collaborative analytical techniques utilized and proposed solutions to improve production allocation. The team conducted a primary investigation using several key investigative techniques and the result led to the identification of two main focus areas to resolve the production allocation issue; a) Improving the existing production allocation method, and b) Improving the current well-test measurement procedure. The team developed collaborative analytical techniques including fit-for-purpose mathematical modelling, specific design for field experiments and advance nodal analysis, which used as a means of identifying the potential root causes in well-test measurement procedure. Following primary investigations, the allocation methodology was updated to include the export meter oil volume readings as an extra step in the allocation algorithm. This helped removing the impact of physical conditions (e.g. weather) from the variations of tank dip measurements. Following implementation in July 2017, results indicated a clear improvement on allocation factors up to 26% on two platforms. Unfortunately, the remaining platforms barely showed any improvement. This was in line with preliminary findings that had identified these platforms as contributing the most to the overall field allocation factor deterioration. On the well-test measurement procedure, the result of collaborative analytical techniques concluded that the current Basic Sediment and Water (BSW) measurement by centrifuge method being deployed in the field was acceptable for crude with a low water content, while it tended to underestimate BSW in wells producing at relatively higher water cuts. With this realization, correction factors were developed and recommended to be applied to the measured BSW to mitigate the measurement uncertainty. The applied correction factors were found to better reflect the actual oil rate from the wells and better match the oil volume measured at platform export meters. The result showed immediate improvements the overall field allocation factor by up to 21%. This collaborative analytical technique to improve production allocation was uniquely developed for a mature oil field producing with extremely high water cut and located offshore in the Gulf of Thailand. Although the collaborative analytical techniques consisted of fit-for-purpose mathematical modelling, specifically designed for the field in question and adaptive approach of nodal analysis, the methods can easily be replicated to other fields, with a number of simply quantifiable potential benefits.