Duchess Kate looked simply stunning in her dove grey Alexander McQueen coat, which she paired with a matching Jane Taylor fascinator.
She showed off a super-pretty half-up hairdo, which also revealed her drop diamond earrings.
Duchess Kate and William received cheers as they walked up the stairs of the cathedral from hundreds of people who'd arrived to get a peek.
They were joined at the service by Prime Minister Tony Abbott and his wife Margie, and they were greeted by the Archbishop of Sydney, Dr Glenn Davies.
The service followed a rare night off for the royal couple, who were spotted enjoying a bit of downtime at a rugby match on Saturday.
The couple spent time chatting and enjoyed the game (New South Wales Waratahs versus South Africa's Northern Bulls - in case you were wondering)
Duchess Kate was looking casual but cool in a grey jumper, jeans and scarf. Wills opted for a royal blue jumper and shirt combo.
MORE ROYAL TOUR 2014 PICS! Keep up with exactly what the Duke and Duchess of Cambridge have been doing during their official trip to New Zealand and Australia with our packed gallery of pics below:
See more from the Duchess in New Zealand here: