An update.
I've opened a new business account at Suncorp after an email from GSB Great Southern Bank saying they don't support 3rd party connections (this reflects their policy). It sucks to feel I've needed go to another bank as I've banked with CUA/GSB for some 40 years and, until now, always backed them as a winning team. Your mileage may vary and there's new 3'rd party connection technology on the horizon later this year in the way of Australia's Open Banking standards. A webpage on GSB as of a couple of days ago, said it's available for personal but not business accounts and I see that Intuit Australia are accredited to exchange data under the scheme but don't know if or when the two shall meet.

With the help of Intuit support staff the connection to Suncorp today was almost first go. I normally use Firefox as my browser but on attempting to connect to Suncorp for the first time I got the same or similar error screen I had before when trying to connect to GSB. The intuit support person then had me open my QuickBooks Online in Chrome Browser (a bit of a pain with a lengthy Firefox securely generated password to write down and type in) and Chrome has worked and my Suncorp account is connected and working. On going back to using Quickbooks online in Firefox it's still working.

I don't know if this helps folk on GSB with business accounts attach if they use Chrome browser, maybe, I haven't tried again. I could change banks as my previous partnership business which banked with GSB has closed and my Sole trader is new so I could be agile and move banks as I'm setting up. As a new electrical contracting business I plan to use Quickbooks with Tradify or Simpro and really need as seamless banking experience as I can reasonably afford in support of the b2b (business to business) access to electrical wholesalers for purchasing such as Rexel or Haymens or Cetnaj as well as for streamlining the usual bank reconciliations.