For Classic Harpoon version 2025.025, Westpac Battleset and HCDB2 170909 Database
There's two hypothesis for this fictitious scenario:
First, a change in USA focus and deployments has left no USN forces in the West Pacific region other than a sole SSN that is in transit after maintenance at HMAS Stirling, Western Australia.
Second, internal Taiwanese political change has led to a swift change in status with PRC who now have use of Taiwanese facilities and have announced a forward military presence in the smaller island countries of the west pacific to enhance their security and to protect infrastructure being provided by Chinese aid.
In an encounter a Philippines Navy Frigate has been sunk by the Chinese, the situation leading up to the sinking is unclear.
A small combined Australian and Singapore naval group is nearby with associated air units conducting combined exercises.
Picture from Wikipedia: Singapore Navy Formidable Class Frigate.