The ArcGIS Online guide includes the following sections:
Note: When Nearmap imagery is loaded in ArcGIS Online, the initial visibility range is set at 1:5,000,000. You might have to zoom in to be able to see the Nearmap imagery.
ArcGIS Online is not Nearmap product, and as such any support you may need should be directed to Esri. Check out ArcGIS Online's support documentation at: https://www.esri.com/en-us/arcgis/products/arcgis-online/resources.