I would advise that you pick a major for the time being that you are most likely to lead into. I'll explain why.
When you pick the flexible program, you do your first semester with the generic lot of courses which every engineer does. In second semester you can do a few electives (usually two, but possibly three depending on what you end up doing). You pick these electives such that it leaves you in a position to continue on with your preferable program later on.
For example, a good elective to pick is ELEC1112 which leaves you with the option to do everything in the School of Electrical, School of Mechanical and Manufacturing and Computer Engineering.
Now at some stage you have to do an internal transfer. You could do this halfway through semester one or at the end of the year after semester two. The issue with this is that there are no guarantees you can make it by an internal transfer. There could be no positions allowing you to do an internal transfer. As a result, this is the main disadvantage of the flexible first year program.
So I advise you to pick something as a safety net. Do you have an idea where you want to lead on to later on? If so, maybe choose that for the time being. If you change your mind, you can still do your same internal transfer as usual with no problems as if you were doing flexible first year.
If you have more questions then ask in here. I strongly recommend you going to Info Day on the 6th too.