Along with casting their ballot the general election, voters were asked to choose whether to retain the existing voting system, MMP.
Only advance votes were counted on election night, and the full result will be announced at a later date.
About 290,000 advance votes were counted, of which 53.7% were in favour of keeping MMP and 42.6% supported a change.
Of the four alternative systems put forward, First Past the Post was the most popular.
