The change was made after part of its Belinau underground mine collapsed in July after experiencing significant unplanned dilution, the company said.
Overall ore production will be lower than originally planned over the next three months, though an annualised production rate of 45,000-55,000oz is expected to be achieved during the first quarter of 2017.
After stabilising the area by filling a void caused by the localised collapse, Sumatra began an interim ore extraction method for ore at levels 5 and 6 of the target zone, developing ore drives between the zones.
Aiming to advance the decline so a more appropriate stoping method can be used, the different method will provide a continuous ore supply to the project's mill.
Sumatra has improved integrated scheduling of jumbo drill and ground support routines and introduced airleg drills in a bid to improve productivity as a result of the change in plans.
The company also plans to reduce the width of the development from 3.6m to 2.5m through the use of two smaller loaders, accelerating ore development and reducing dilution.
Sumatra also redesigned its Berenai-Nuri open pit and changed its plans for the Siamang open pit after encountering additional intermediary veins.
The revised Berenai-Nuri plan brings forward 83% of the project's contained ounces over the coming year by delaying the waste cutback to the north, the company said.
Shares in Sumatra were unchanged at A3.8c in afternoon trade.