The poem starts by setting the scene. The speaker creates a vivid picture of 1960s Sydney, name-checking some key establishments: Repins was a coffee shop; Lorenzinis was a wine bar; Tattersalls was (and still is) a sporting, social, and business club; and the Greek Club was a restaurant. This is clearly a bustling place. The diacope of "goes round" in lines 1 and 2 also helps the reader get a sense of the speed with which news travels around Sydney, suggesting that, on the level of information at least, the people here are pretty connected.
The speaker also implies that many of these spaces are dominated by men—"men" is repeated twice, in fact. Likewise, the "scribblers" (the stock-brokers) at the Stock Exchange would also have been men at the time. Though the poem does refer to women (and children) later on, this opening stanza, combined with the fact that the person "crying in Martin Place" is a "fellow," makes it clear that the poem concerns thematic ideas of masculinity—what it means to be a man and, specifically, to express emotion.
This first stanza also plays with the reader's expectations. The title suggests that the poem has something to do with a rainbow, and the gossip and excitement that travels around Sydney society like electricity suggests that the rainbow has arrived in the poem. But what awaits the reader in line 6 is the reveal that the commotion isn't caused by a rainbow at all, but rather by someone crying who refuses to "stop."
Notice how the poem's syntax (the arrangement of its words and phrases) creates the surprise. The first five lines are one long sentence that builds drama as it unfurls down the page until the colon that abruptly halts things at the end of line 5. Something has suddenly stopped the hustle and bustle of the city. Line 6 then explains what, exactly, has put society on pause—not a rainbow, but rather an outpouring of emotion:
There's a fellow crying in Martin Place. They can't stop him.
That said, it's perhaps not really a true pause and more of a shift in attention. Suddenly and seemingly out of nowhere, this crying man seems like the most important event that the inhabitants of Sydney can focus on.
After the full-stop caesura that follows "Martin Place," the poem reveals that people have tried and failed already to stop the man from crying. The placement of this phrase—"They can't stop him"—after the stop of a caesura is somewhat ironic, the brief pause in the poem quickly aborted to indicate that this is an ongoing situation. It's also worth thinking about what kind of event usually creates this kind of city-wide response. By and large, it's usually some kind of danger that so drastically alters metropolitan daily life—already, then, the poem suggests that the man represents some kind of threat merely though the act of crying. This heightens the sense that the man—and his action—represents something alien and disturbing.
The stanza also makes use of some subtle but effective sound effects. Alliteration in "round Repins" (line 1) conjures the sound of verbal excitement (as though the /r/ sound is a rumor going round), and the internal consonance of "murmur" achieves something similar. The clustering of consonance in line 4—the Stock Exchange scribblers forget the chalk—gives the impression of hurried economic activity.