Hi Fuchsia
Having remained very interested in China and SE Asia since my first vist there in Jan 2004 and many times every year after that I believe the Mango pancake turned up in Sydney from about the late 90s. ( there are also Durian pancakes now ! )
But I guess we really need to ask the propietors and chefs in Sydney who pepare the Yum Cha. I’ll do some more research for you.
Yum Cha in Australia has been driven by Cantonese migrants to Australia. As you know Yum Cha is an ”institution” in HK and in every SE Asian city where there is a big Cantonese population eg Malaysia.

I have eaten Yum Cha in every major city of SE Asia including Jakarata, Kuala Lumpur and Sinagpore and mango pancakes could be from there !

The variety of Dim Sum in Sydney is just as good or better than HK in my experience so my guess is that Sydney Dim Sum chefs are not just from HK but Malaysia and other SE asian countries too.

Cantonese have been coming to Australia since the 1850s but I believe Yum Cha first appeared in Sydney in the late 1970s or early 1980s with the arrival of a new wave of immigrants from HK after the relaxation of the immigration rules for Asians to Australia since the 1970s. ( end of the so called ”white Australia policy”… 2 Wongs dont make a white ! )

I beleve Sydneys population is now almost 10% ethnic Chinese with the biggest group now from the PRC.
There are Chinese immigrants in Australia from all the SE Asian countries with large ethnic Chinese populations such as Indonesia, Thailand, Malaysia, Singapore and they have brought all their regional and regional Chinese cuisines with them. It makes for very exciting Asian eating options in our cities now.
Maybe the Mango pancake originated in one of the SE ASian countries such as Malaysia.

We now have at least 6-8 large ”Chinatown” areas in Sydney and numerous Chinese daily newspapers. All the suburbs seems to have Chinese grocers.
One area is now called Xiao-Shanghai and is home to many good Shanghaianese restuarants.

Hope you can visit Sydney again some time.