Juice the oranges and measure out 2 cups of freshly squeezed orange juice.
Chop the ginger and bruise the lemongrass with the back of a knife.
Put the water, orange zest, ginger, lemongrass and sugar in a large, heavy-bottomed pan. Simmer, stirring until the sugar has dissolved and the ginger and lemongrass has infused for at least 15 minutes.
Remove from the heat, stir in the orange juice and then bring back to a simmer for 2 more minutes. Take off the heat once again.
Skim off any scum from the surface, strain through a fine-mesh sieve, then pour into warm, dry sterilised bottles and seal.
Keeps unopened for up to 4 months. Once opened, keep in the fridge and use within 4–6 weeks.