A pensioner who pioneered a $450 million water park development on the Sunshine Coast but was left with nothing to show for it has won a key court battle against a group of investors that includes champion surfer Julian Wilson.

Water park expert Arthur Downing, 73, spent five years finding and securing the site on Steve Irwin Way at Glenview before agreeing to sell his interest in the scheme in 2015 in exchange for a $1.25 million “assignment fee”.

An artist's impression of Sanad Capital's planned Sunshine Coast water park development.

But Mr Downing was never paid, prompting a series of legal actions in the Queensland courts.

Dubai-backed Sanad Capital has since obtained planning permission from Sunshine Coast Council for a water park attraction, four-star hotel and retail outlets on the 24-hectare site near Australia Zoo.