The four central rooms of Koonoona, the earliest section of the building, were built in the late 1860s by the selector, John Caldwell. The building comprises a pole frame faced both inside and outside with close set horizontal saplings of Melaleuca ericafolia, packed with pug. The roofing of the older part is of galvanised iron tiles, of which the majority are of the Morewood and Rogers type with the minority, possibly added later, of Walker's G (anchor) O brand (Gospel Oak), not previously reported in Australia. There is also a skillion milking shed roofed and clad on one wall with Morewood and Rogers tiles. There is no other vernacular structure of comparable age in Victoria which remains today so well preserved. The cottage illustrates the lifestyle of early settlers in colonial Victoria and has important associations with the pastoral era on the Mornington Peninsula.