I've taken a look at the contour map for this area. This particularly spot is the lowest point for southern draining streets (northern draining streets would flow to big drain easement through Browning Cres). Downslope dead-end streets have twin concrete drain covers above the kerb (on the nature strip). Twin openings caters for extra flood water that pools in the dead-end. The difference with Peerless Ct is that is also has two grates on the road. I assume this is because there is a major stormwater junction here (possibly receiving Oakley drains), so addition pits or pipe diameter increases occur here, therefore additional maintenance access requirements. If there is a sump pit for pollutants, this would be the ideal spot before water discharges to Milleara Drain.
Also check out Googly Maps laneway. Looks like another grate at the other end of the walkway. Everything may connect around here so good place to explore the underworld.
Watch out for clowns though.