Maimai Siding is not too far south (east) of Tawhai, which is about 10 km south (east) of Reefton. The original Maimai station was at 31.2 miles and it was opened 1891 and served a sawmill. There was a ballast siding at the site which apparently closed 1914. I haven't located if there was a pit served by this siding and if so, where that pit was. The station closed completely in 1980.
The present siding points are a little south of the old station and were installed in 2008 when track was laid with a loading bank to serve coal traffic from the Giles Creek opencast mine. As detailed in a post on Enzed Transport four years ago the mine was first developed in 1984 and has grown to reach a 500,000 tonne annual output. Birchfield Coal which owns the mine wanted a siding put in to rail the coal out to customers. This siding was originally shorter than now and only served the loading bank which was made out of old steel highsider wagons. The mine is 15 km to the north making it nearer Reefton, bringing the coal out via Maimai avoids the need to bridge the Inangahua River.
When Birchfield won the contract to supply the Fonterra dairy factory at Darfield the site was redeveloped and the siding was extended considerably. The coal is loaded into open topped containers at Giles Creek which are taken by truck to the siding where they are lifted onto container flats. There are a few customers for whom coal is roadbridged using these containers, mostly various Fonterra facilities. For example the same system is used to load coal from Ohai in Southland which is railed to Temuka where the containers are roadbridged to the Clandeboye dairy factory which has no rail siding. The advantage of this system is that the special unloading facilities needed for the hopper wagons that are used by Solid Energy aren't required. The containers can be lifted off the wagons and then tipped to unload at the other end.