Hey, I'm not doing neuroscience but I am doing a majority of the papers that neuroscience majors do.. as greenglacier said, the degree actually turns into "neuroscience" in 3/4th year before that its just a mix of papers from different departments.
In terms of difficulty, I dont know how hard auckland papers are, but if their similar to HSFY difficulty, if you did well in your first year (A/A+'s) then its not "impossible" to do well in 2nd year (sorry this is all ive done so far) - by this I mean if you continue to study hard theres no reason you wouldnt get A/A+ in 2nd year - the number of lectures per week are much less and the work is presented much more coherently (I found anyway - I found in 1st year some concepts were displayed pretty vaguely). Obviously the content is "harder" as it builds from 1st year, so if you "slack off" I'd think 2nd year would be pretty hard.
As for 3rd year, from heresay it is more difficult - this is because you have to do seminars/speeches/presentations etc which require a large effort I wouldn't say its impossible to do well in 3rd year either, but once again if you continue to actually work hard theres no reason not to atleast aspire for A/A+'s.
So I guess a rule of thumb for "how hard", for an A+ at otago you would need to = go over all the lectures, memorise each slide word for word (overstatement obviously lol), and some 2nd year papers are quite lab heavy (pharmacology etc) so you would also need to be willing to put in effort every week and possibly be willing to do work for others (your "partners") as its quite common to get into groups where there are people not wanting to put in the same effort (sorry brought back bad memories

). I guess I could add that for some papers getting A-/A is alot harder because the internal work is so heavy and subjectively marked, thats another key different between 1st year and 2nd year (in otago anyway, 1st year internals are mainly just mid-terms etc not essays and presentations).
If you get medicine in first year please take it, don't put yourself through something you simply don't have to do, no one in BSc aiming for medicine/dentistry "wants" to do it. It's 2 more years of struggle.