Don't watch it now but it's a common thing with Australian stuff. Whereas we'd write out a character if the actor was ill or something and couldn't film, they just get a new actor for the same part. It's usually only temporary - maybe this is.
It is only temporary. This question has been asked about half a dozen times! The actress was in hospital with pneumonia and a collapsed lung and because most of the script had been written about Libby's wedding plans they replaced her until Kym Valentine recovered. It was for about 5/6 weeks.