This is War Watch Mk1, a Billabong Surftime, 38 mm with a 20mm lug width.
I purchased this in 1994 while still at HMAS Cerberus, which is the main RAN training establishment. It originally had a Velcro strap, which made it easy to take off and put on the Anti-Flash clothing when we would go to Action stations. Anti-flash protective equipment consists of hoods and gloves designed to shield personnel from the flash and heat of explosions, such as those from shell fire.
This watch was eventually replaced with the Mk2 in 2013, after I grew tired of receiving shocks from it when reaching down to the back of the equipment I maintained. It is also why it is scratched up so much. The bezel is still functional, though it does require a little more oomph than it used to, and was perfect for timing between defect reports when at action stations.