Prior to the changes made in September 2010, members of Parliament were not considered designated public office holders. While initial registration had to be there, those encounters didn't necessarily have to be reported, unless the member was a designated public office holder. Prior to September 2010, it affected only ministers and their staff.
The advice changed with lobbyists and public office holders. Now, if you go to a lobby day, you're basically agreeing that you may be lobbied at that event. You may end up communicating with someone, but it's not about a registrable activity. You would not need to register that encounter.