A friend of the bride, Dragana Ogorelica, said they got 'the shock of our lives' when they saw the star standing there.
And Butler didn't disappoint the women; posing for several cheesy snaps, that Dragana later tweeted.
'We though it was a stripper, because it was a hen's party. But then we saw [Gerard] standing on the boat. We all just started screaming.'
Gerard then told his management team to leave because he was going to stay and party with the girls.
The bride, Maja Nogic, was apparently ecstatic about the meeting - and quickly text her fiancé (who was on his stag weekend) to tell him who she'd met.
Her fiancé replied 'That's it, I'm going to go and find Jennifer Hawkins.'
Friend Dragana said that all of the girls were in love with Gerard's accent, saying: 'It was so beautiful. You couldn't understand him half the time, but you didn't care because it was just gorgeous.'
The 300 star is currently in Sydney as he prepares to film Gods of Egypt, he had hired the boat on Friday morning to look at some properties in the bay.
What a lucky group of ladies!