Thanks for all the guess-work in everyone's replies but ive been in contact with Christine Mccafferty the Archive Manager from DIAGEO. She emailed me with:
Like all distillers John Walker & Sons dabbled in the full range of products, not just the Scotch Whisky that we are so familiar with. Walker's sold brandy, port, wine and rum in the past and a lot of these products were bottled in Australia for that market. Frigate Rum is one of these products and by looking at the design of your bottle, it could date back to the 1940s. I have come across a few of these bottles over the years but not a huge amount so they are quite collectable.
Sorry I can't attach a photo. I have an apple mac, is this the problem Jojo?