"Hey all you cool cats and kittens, it's Carole Baskin at Big Cat Rescue," she says on film.
"Hi Rolf Harris – all your kids wanted to get together and tell you that you have really touched them and that they love all that you have done for them.
"I hear there's a lot of great stories about you and your best friend, Jimmy Saville – can't wait to hear those.
"Happy birthday Rolf."
Rolf Harris was one of Australia's most successful entertainers before being sentenced to five years and nine months behind bars in June 2014 for a dozen indecent assaults against four teenage girls between 1968 and 1986.
He only served three years of his sentence before he was released on parole in May 2017.
Jimmy Saville was a hugely successful BBC personality for decades – but after his death in 2011, hundreds of allegations of sexual abuse were made against him.
Last week, Armstrong pulled a similar prank on Mitchell, who was seemingly unaware of Ivan Milat's crimes.
"Hey Ivan! Not many people pick up hitchhikers anymore, but you really helped out the backpacker gang and you belong in 7th Heaven. So thank you, we need more people in the world like you Ivan … Milat, I hope I'm saying that right," she said in the clip
"Sending you lots of love and thank you again for being such a big fan and for just being awesome and being you."
That joke divided opinions, with many people finding it hilarious while others questioned whether it was too disturbing.
