A 21-year-old water polo instructor has been found dead at a prestigious Sydney CBD school, sparking a full-scale investigation.

Lilie James was discovered with serious head injuries by police after midnight on Thursday in a campus bathroom.

Authorities have launched a manhunt for her colleague, Paul Thijssen, who James had reportedly dated in the lead-up to her death.

Australian correspondent Murray Olds says the school will be closed to all pupils until Monday.

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