Chatham Island - EPSG:5771

WKT

VERTCS["Chatham_Island", 
	VDATUM["Chatham_Island"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["Chatham_Island", 
	VDATUM["Chatham_Island"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5771"]]

Data from @esri/proj-codes

{
  "wkid": 5771,
  "latestWkid": 5771,
  "macro": "PE_VCS_CHATHAM_ISLAND",
  "name": "Chatham_Island",
  "wkt": "VERTCS[\"Chatham_Island\",VDATUM[\"Chatham_Island\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"Chatham_Island\",VDATUM[\"Chatham_Island\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5771\"]]",
  "description": "Chatham Island",
  "authority": "EPSG",
  "version": "6.15(9.3.0.0)",
  "deprecated": "no",
  "areaname": "New Zealand - Chatham Island onshore",
  "extent": {
    "slat": -44.18,
    "nlat": -43.67,
    "llon": -176.92,
    "rlon": -176.2
  }
}