By tournament:

1920 Maarist School Sevens

The list may be subject to change if more rugby union sevens tournaments are discovered.

Maarist School Sevens

From The Official History of the Melrose Sevens by Walter Allan:

The country became a British protectorate in 1914 under the administration of New Zealand, and within 6 years the first sevens were being played at Maarist School.