Of course within each area, the government will decide how to spend the funds. For example, just because you distribute your taxes to Foreign Affairs doesn't mean that more will be spent on processing immigrants and assylum seekers, it might instead fund an overseas trip for a member of parliament. Likewise, money distributed to social support could just as easily be used for paying benefits as it could be for hiring private investigators to expose fraudsters.
Therefore the dilligent taxpayer should evaluate how the government is planning to use funds in each area before deciding which area(s) to nominate for their taxes to go to. (And of course the 50% up to government discretion is to allow for the government to prop up less popular areas - such as administration costs for example.)