Here are a few tips for dog friendly lakes near Graz. (Check their websites for updates in case of changes to dog rules). Also this is a useful book with dog friendly hikes including access to water.

Murfeld near Graz – On the dog meadow (Auwiese) there is a fenced pond where dogs can cool off to their heart’s content.
Erlaufsee – dogs have their own section (1000 m²) here (from the second footbridge to the meadow).
Packer Stausee– Dogs are allowed to use the free access to the lake areas, but not the public bathing area.
Hirzmann reservoir in Edelschrott – Dogs are also allowed here, but not on the public jetty.
Badesee in Gaishorn – dogs are allowed to swim in marked areas
Hundewiese Auwiesen Liebenau
Rudolfsee in Hirschegg
Stainz – Dogs are also allowed in the water at the TSV-Stelzl leisure facility
Stausee Soboth
Altausseer See – you can swim with your dog away from the official lawns.
Grundlsee – allowed to swim apart from the official bathing beaches
Salza reservoir in Bad Mitterndorf.
Leopoldsteinersee in Eisenerz – Dogs are not allowed on the beach, but are allowed in the water everywhere else.
Podolerteich in St. Blenden
Teichalmsee
Natural pond Grosshart
Hebalmsee
Ödensee in Pichl-Kainisch
Sulmsee
Das Sulmbad Zur Steinernen Wehr Kaindorf an der Sulm.
Großlobming (near Knittelfeld)
Reservoir Soboth
Gleinstätten natural bathing lake – Dogs have their own access to the water here
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