Hi there neill,
Plant propagation labs grow their plants in sterile jars with nutrient agar at the bottom to sustain the plants. These sterile containers are allowed into New Zealand without passing thru quarantine as long as the plants it contain are allowed by MAF.
The aquatic plants may be planted straight from the sterile jars , after cleaning the agar off the roots, and they grow as if nothing abrupt happened. this is actually better than growing them emersed to get good growth and then submerging them.
By the way, the plants in the sterile jars grow their submerged form even if there is no water inside. Knowing this, it is easy to select which ones will look good in the aquarium.