diff --git a/port-authority-nsw/README.md b/port-authority-nsw/README.md
index 145a288..4f05703 100644
--- a/port-authority-nsw/README.md
+++ b/port-authority-nsw/README.md
@@ -5,6 +5,23 @@ Download latest time series data from Port Authority of NSW and update local cop
 
 Only one observation is available at any given moment. This script must be run regularly to obtain a complete time series.
 
+## Chart datum levels
+
+Water level observations are reported in chart datum. The offsets from AHD are below:
+
+| Location | Chart zero (m AHD) | Stations                                  |
+|----------|-------------------:|-------------------------------------------|
+| Sydney   |             -0.925 |  Brotherson Emergency Response Jetty Tide |
+|          |                    |  Caltex (Directional) Current             |
+|          |                    |  Fort Denison-Primary (Nth end) Tide      |
+|          |                    |  Fort Denison-Secondary (Vegapuls64) Tide |
+|          |                    |  Circular Quay ADCP Current               |
+|          |                    |  Balls Head Current                       |
+|          |                    |                                           |
+| Yamba    |             -0.895 |  Middle Wall (Vegapulse WL61) Tide        |
+|          |                    |  Goodwood (Vegapulse WL61) Tide           |
+
+
 ## Station locations
 
 | ID | Description                               |