In 1914, a railway branch line from the North Auckland Line was surveyed to Waipu to serve agricultural activity in the area. Construction of the line was delayed due to World War I, but by 1920, 25 men were employed in the construction of formation. However, by 1924, private motor vehicles were becoming more common and railway lines to sparsely-populated rural areas accordingly became less necessary. Due to the lack of significant industrial activity in the Waipu area, the branch line was no longer seen as economic and construction was cancelled before any rail tracks were laid. However, a new railway line, the Marsden Point Branch, is currently proposed for construction and will follow a route similar to that of the abortive Waipu line.
The real question of course is the route it followed and how much formation work was completed prior to cessation of construction. No route has been found on maps of the period. It seems likely the route would have largely followed State Highway 1 from somewhere near Oakleigh. The Marsden Point route can be found in a Flickr album here.