The quid pro quo

If a top Aussie publisher has requested a rewrite on my novel, is this the time to call an agent?

It's time to get some advice, whether that's an agent or someone else who might have some knowledge about the industry (for example, a published author or someone at your state's writers' centre - links are at the right for those). It sounds as though the publisher has not promised anything much in exchange for the rewrite - they're asking you to do what may amount to a lot of work presumably in exchange only for another reading by them, not for a promise of publication. If you want to do this work anyway - you were planning to do another draft, and their feedback is just exactly what you wanted to hear - then that's great, because even if they don't end up taking the novel, you have a new draft to send to agents and publishers. But if you're only doing this work because they're asking you to, it's time to get a second opinion.