diff --git a/apps/overlays/au-syd1/externaldns/values.yaml b/apps/overlays/au-syd1/externaldns/values.yaml
index 107e83e..865aa0c 100644
--- a/apps/overlays/au-syd1/externaldns/values.yaml
+++ b/apps/overlays/au-syd1/externaldns/values.yaml
@@ -48,6 +48,10 @@ extraArgs:
   - --rfc2136-port=53
   - --rfc2136-zone=k8s.syd1.au.unkin.net
   - --rfc2136-zone=200.18.198.in-addr.arpa
+  # Synthesize PTR records into the reverse zone for every A record managed in
+  # the forward zone (the reverse zone + resolver forwarder exist since #288;
+  # without this flag nothing writes PTRs).
+  - --rfc2136-create-ptr
   - --rfc2136-tsig-keyname=externaldns-key
   - --rfc2136-tsig-secret-alg=$(EXTERNAL_DNS_RFC2136_TSIG_ALGORITHM)
   - --rfc2136-tsig-axfr