The episode's title is "What Do You Really Want?", it's the sixth episode of season 2. Tabrett plays Kate, the sister of Brett Tucker's character. He was in Legend of the Seeker too, in Perdition, he played the new Seeker in Richard's nightmare. You might also know Brett from one of his guest starring spots in American tv shows, or from McLeod's Daughters, the very popular Aussie show, he played the vet for a couple of seasons.
Tabrett shot the episode in April. Here's her Twitter post about it:
2014 April 5 - "Hard day in the office working with the super talented Brett Tucker #harry on #Mistresses #ABC #lovemyjob"