At the last count, the Nationals candidate received more than 60 per cent of the vote, recording the highest byelection swing in NSW history.
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Mr Marshall was congratulated by Deputy Premier Andrew Stoner, who said the Nationals now represented more than 90 per cent of the NSW landmass.
The party will also have 19 members in the lower house, just one fewer than the Opposition.
Mr Marshall said the result was a sign that the Nationals "are back and we're in it for the long haul".
He also made reference to the federal election in September, claiming the result would send New England MP Tony Windsor "a very strong message".
Mr Marshall's closest rival in the byelection, independent Jim Maher, polled about 14 per cent of the primary vote, while Labor candidate Herman Beyersdorf received about 9 per cent.