Well the advantages for foreign owned ship yards in Australia depending on how you look at it. The first being a 'peace time' (i.e. not a World War). The second being a 'non-peace time' (i.e. a world war or Australian mainland being threatened.
The first being the current period we are experiencing, we have a known demand and potent for more work due to gaining experience through the current plans, this allows multiple ship builders to compete for Australian local builds and high quality producing due to the experience of our current companies and industry. This is good for Australia because it creates a competitiveness for companies to low their prices and/or shorten their time frame of production, both good outcomes if you are trying to build ships (which Australia and other nations are currently doing). It can also create a competent maintenance force for Australian built ships.
The second way to look at it is in a war time environment, it could create a new ease of maintenance for Australian built ships going in and out of combat and needing their own fair share of remediation work, upgrades or damage repair. It also can create fall back bases in the case of Australia being invaded or fleet bases being destroyed (Yes i understand very unlikely and hard to do but it is better than nothing). Foreign owned shipbuilders can also bring their own hardships i.e. planning and communication errors, which can effect productiveness. Foreign owners can also take their investments back home, if their company is in danger of being over run or going into financial woes. It could also bring Australian jobs and new skills in our workplace's. Another possibility is of (only for imaginative purposes) say Spain was over run, Navantia would immediately move their base of operations and other parts of their company to Australia due to safety and current work in Australia.
These are some of the possibilities of foreign shipbuilder setting up shop in Australia , some possibilities many never come true.
I hope this answered your question Hairyman