New Zealand has used an MMP system since 1996.
In the 2017 general election, 27.33% of voters split their vote (that is, used their candidate vote for a candidate of a party different from that for which they used their party vote).
In general, splitting votes is much more common for supporters of minor parties. Of those who voted for National or Labour (the two major parties), only 14.13% and 22.94% of voters respectively split their vote. In comparison, of those who voted for the Greens, NZ First, or ACT (the three minor parties who hold seats), 66.62%, 59.06%, and 81.27% of voters split their vote.
This makes sense, as typically general electorates are won by either National or Labour (unless a major party endorses a smaller party for a particular electorate). A supporter of a minor party will typically vote for either the National or Labour candidate with their candidate vote, and vote for their preferred minor party with their party vote.
These figures is broadly in line with previously elections, with 31.64% split in 2014 and 30.70% split in 2011.