Remove leaves from celery and set aside. Cut 3 stalks celery into thick pieces and place in a large saucepan. Add 2 litres water, 3 tablespoons vinegar and 1 tablespoon salt. Bring to a boil. Place lobster tails in saucepan, cover and cook over low heat 15 minutes. Leave in liquid to cool.
Thinly slice remaining celery stalks. Halve tomatoes, remove seeds and dice. Slice spring onions. Finely chop celery leaves. Combine celery leaves and stalks, tomatoes, spring onions, garlic and parsley in a bowl.
Whisk olive oil and remaining 3 tablespoons vinegar until combined; add salt and pepper to taste. Remove lobster tails from liquid and remove flesh. Discard shells and cooked celery.
Cut lobster flesh into small slices. Place in a bowl. Pour 4 to 5 tablespoons cooking liquid over lobster and toss to coat lobster. Arrange salad on individual plates, drizzle with vinaigrette and top with lobster pieces.