Which Oceanian country's flag is entirely red with a white canton containing a red cross? Tonga. The national flag has a red field, with a white rectangle in the upper hoist corner bearing a red couped Greek cross.
The design was adopted in its present form in 1875, when it was formally written into Tonga's constitution. The red cross reflects Tonga's Christian heritage; white traditionally represents purity, while red is associated with sacrifice and the blood of Jesus in the flag's symbolism.
Tonga's earlier flag placed a red cross on a white field. That arrangement looked almost identical to the emblem of the International Red Cross, adopted in 1863. To avoid the resemblance, Tonga moved the cross into a white canton on a red field, creating the distinctive design used today.
Tonga's constitution states that the national flag cannot be altered. This helps distinguish it from nearby Pacific flags such as Fiji and Samoa, which also use red, white, or blue but have different layouts and symbols.