900/25/16CA is the adaptor that can be used for 25 x16mm ducting to 20mm conduit.

Please go to the following links below and check under the Documentation Tab for more information.
Australia - https://www.clipsal.com/products/detail?CatNo=900/25/16CA&itemno=900/25/16CA-WE&backto=/search?q=900/25/16CA&tab-document-1=0
New Zealand - https://www.pdl.co.nz/products/detail?CatNo=900/25/16CA&itemno=900/25/16CA-WE&backto=/search?q=900/25/16CA&tab-document-1=0