Likely a variant of Talia, from Hebrew roots meaning dew from God or gentle dew.
Tahlia is most often treated in modern usage as an Australian English variant of Talia. One line of interpretation connects that underlying form to the South Australian place-name Talia, thought to come from an Aboriginal language and often glossed as something like "near water." Another association, especially outside Australia, links it in people's minds with the Hebrew Talia, "dew from God."
That means Tahlia sits at an interesting crossroads: part spelling variant, part phonetic refinement, part meeting point of different naming traditions. Its rise has been relatively recent, and that modernity is part of its style. The extra h gives the name a slightly more ornamental, contemporary look than Talia while leaving the pronunciation largely unchanged.
In Australia and Britain it has had a noticeable but never overwhelming presence, which helps it feel familiar without becoming generic. Public bearers such as cricketer Tahlia McGrath have strengthened its image as a distinctly modern, confident choice. Culturally, Tahlia belongs to a class of names that feel fluid, feminine, and nature-tinged.
Water, dew, and softness gather around it, even when people disagree on the precise root. It has evolved less through ancient legend than through modern spelling practice and regional taste, yet it still carries poetic associations. The result is a name that feels contemporary but not flimsy: bright, graceful, and a little mysterious, with enough ambiguity in its history to invite conversation.