is a visa required to travel from Kowloon to Shenzhen for day trip

Yes, but Australians can apply for visa on arrival in Shenzhen. Please visit this page for more information:
https://www.travelchinaguide.com/embassy/visa/service.htm

Hi Debbie,
You do need a visa for travel to Shenzhen but you can acquire this at the Hong Kong (HK) / Shenzhen (SZ) border. You clear HK customs and walk through the terminal, keeping to your left hand side you will see escalators going up to the visa office. You fill out a basic form as the visa application and hand it to the officers. The granted me the visa in about 10 minutes and then cleared immigration to SZ. I went in December last year, I think it cost 170 RMB for the visa. You can only visit SZ and I think it is only valid for 5 days.
Happy Travelling.

Additionally you can only obtain this visa at the Luoho border crossing. You can catch the subway there on the light blue line (East Tsim Sha Tsui) or (Mong Kok East) You Could also change trains on the Kowloon Tong interchange and head to Louho border crossing. I do not think any other border crossings will grant you the on the spot visa.