You would need at least a 70 average across 4 3rd year actuarial units, however there is 1 3rd year unit worth 25 credit points so if you get a 70+ in that unit then it counts as 2 units for that requirement i.e. you only need to get another 3rd year unit with a 70.
From what I've heard, it is difficult to maintain a 70 average in actuarial. You really need to put the effort and work into your units to do well and have an aptitude in math. Also, I think that you still need a fairly solid average to get accepted into honours, although I don't know what the figure would be - probably at least a 75? As that shows you've been able to achieve the average for the units required for accreditation and being eligible for honours.
If you're set on this path be prepared for a lot of work!