He says that she was "running around backstage looking for somewhere to do the coke" 10 minutes before she was due on stage.
"She pulled me into the changing rooms and when she saw that there was no toilet seat or other surface to cop up the cocaine she said, 'F**k it, the ground, let's do it off the ground,'" Mr Maselle told The Sun.
"I thought, 'My God, you will collapse.' But she laughed and went straight out on to the catwalk and was seamless."
Afterwards, he said, she went through several more grams as she partied with a multi-millionaire businessman, a world-famous rock star and a host of A-list models.
A chauffeur who drove her around in South Africa in 1997 also said she would take lines of cocaine in the back of the car while going from the hotel to shops.
Mr Maselle added: "Kate needs to realise what is real in her life, what is good and what is bad - and stay off coke."