Prior to coming to the Bar, Sebastian practiced at Beatty Hughes & Associates and before that King & Wood Mallesons. He has worked at the front end of government infrastructure projects, in construction and insurance litigation, and has particular experience in the NSW Land and Environment Court. He has acted in proceedings in Classes 1, 3 and 4 of the Land and Environment Court's jurisdiction, as well as in the Equity Division of the NSW Supreme Court.

In 2018, Sebastian worked as a tipstaff to the Hon Justice Lindsay in the Equity Division of the Supreme Court of New South Wales.

Sebastian holds a Bachelor of Laws (First Class Honours) and Bachelor of Business Administration from Macquarie University, as well as a Master of Laws from Melbourne University. At Macquarie University, he was awarded the Dean's Award for Outstanding Academic Achievement in Law and the Dean's Award for Outstanding Contributions to the Law School twice.

Sebastian accepts briefs in all areas of law.