The Manaia Branch was a line of about 9 km in length that was developed in the early 1920s from Kapuni, on the Opunake Branch which was also under development at the time, to Manaia, more or less directly to the south. The only official references to it found so far are the following paragraphs from the Public Works Statement (D-1) of the Appendices to the Journals of the House of Representatives (AtoJs) for 1923.
Kapuni Branch: […] The opening-up of the ballast-pit at the Kaupokanui River on the Manaia Branch line has been completed. The access to this pit comprises the laying of 30 chains of siding and branch line, together with a bridge across the Kaupokanui River.
Manaia Branch (0 m. to 5 m. 49 ch.; length, 5 miles, 49 chains).—The formation and culverts on this section have been completed with the exception of several cuttings and banks which will be widened by work-train, and everything is in readiness to start platelaying.
And there rests the matter. I have not found anything to confirm the line was ever completed or opened. The Quail Atlas tends to confirm the idea that the line was never completed beyond the formation works. Here are the three maps which I have been able to draw showing the route from originals held in the Alexander Turnbull Library collection.