Green ammonia has received increasing interest for its potential as an energy carrier in the international trade of renewable power. This paper considers the factors that contribute to producing cost-competitive green ammonia from an exporter's perspective. These factors include renewable resource quality across potential sites, operating modes for off-grid plants, and seasonal complementarity with trade buyers. The study applies a mixed-integer programming model and uses Australia as a case study because of its excellent solar and wind resources, and the potential for synergy between Southern Hemisphere supply and Northern Hemisphere demand. Although renewable resources are unevenly distributed across Australia and present distinct diurnal and seasonal variability, modelling shows that most of the pre-identified hydrogen hubs in each state and territory of Australia can produce cost-competitive green ammonia providing the electrolysis and Haber-Bosch processes are partially flexible to cope with the variability of renewables. Flexible operation reduces energy curtailment and leads to lower storage capacity requirements using batteries or hydrogen storage, which would otherwise increase system costs. In addition, an optimised combination of wind and solar can reduce the magnitude of storage required. Providing that a partially flexible Haber Bosch plant is commercially available, the modelling shows a levelised cost of ammonia (LCOA) of AU$756/tonne and AU$659/tonne in 2025 and 2030, respectively. Based on these results, green ammonia would be cost-competitive with grey ammonia in 2030, given a feedstock natural gas price higher than AU$14/MBtu. For green ammonia to be cost-competitive with grey ammonia, assuming a lower gas price of AU$6/MBtu, a carbon price would need to be in place of at least AU$123/tonne. Given that there is a greater demand for energy in winter concurrent with lower solar power production, there may be opportunities for solar-based Southern Hemisphere suppliers to supply the major industrial regions, most of which are located in the Northern Hemisphere.