It was first seen in Utopia, and the subsequent episodes The Sound Of Drums and The Last Of The Time Lords.
Louise had explained to us that for the series two double episode, The Impossible Planet and The Satan Pit, she had chosen a new tie (Tie Six) for a one-off use in that story, thus reducing the the chance of over exposure of her more favourite ties.
So when it came to the season finale story arc to series three, she put the same principle into practice, using a brand new tie that after this story was never seen again.
Louise explains the background to this unique tie.
Thanks to Kevin Coppa for giving the interview clips the polished look, and enormous thanks to Louise Page for sharing her memories of dressing the Tenth Doctor with us all