I understand it’s not necessarily bad form to query more than one agent simultaneously, but if an agent, having read a submission, decides to represent someone, how might s/he then react to the news that the submission currently sits on the slush pile of one or more publishing houses?
Most Australian agents would probably expect that authors are sending things to publishers at the same time as they send them to us, since some publishers do accept submissions from the public. The kosher thing to do is mention in your query letter - to all parties - that you are submitting the manuscript to other agents and publishers, just so there are no surprises. If the agent doesn't like that, then they don't, but it doesn't really make sense for authors not to take advantage of publishers' open submissions if they're offered. Having said that, if all the publishers have said no, it makes it less likely that I'll offer to represent the author because I won't have as many options for submission.