The call, "I've been robbed" from a distressed Opera House jeweller, sent two Australian Navy Cadets to "full alert" last week.

Brother and sister, AB Peter O'Hehir, 18 and SMN Katrina O'Hehir, 17, from TS Lismore did not tackle the fleeing thief but obtained an excellent description of him.

They also went to the victim's aid comforting her and remaining with her until security guards and police arrived.

The robbery occurred on one of the retail shopping levels of the Opera House and involved a shop selling opal-based jewellery.

The thief asked to see rings then snatched some from the display and ran off.

At the time Peter and Katrina, in Sydney as part of the Youth Sea Day program, and out sightseeing, were close by when the robbery occurred.

The thief had to swerve past Katrina as he sprinted away and as a result the cadets obtained a good description of him.

The pair provided the description to the security guards and police.

The teenagers, residents of Goonellabah, and in Years 12 and 11 at Kadina High School, were later praised by their cadet officers for their presence of mind and follow-up action.