We spray the lambs as soon as we suspect or observe that flies are around, here usually in May, so April down there in the tropics wouldn't be too early. If it's a muggy, humid heat then almost certainly they will be around - keep your eyes peeled
Also keep the bums well dagged on ewes and lambs alike. We don't usually spray the adults until they have been shorn, because I use the fleeces for spinning and Crovect etc are all to be avoided for prolonged contact. If we do have to spray before shearing then any contaminated fleece would not be used for spinning. Our Hebs don't tend to be daggy but some of the Soays do - seems to be an inherited factor, so we watch those ones carefully.
Also remember sheep can get struck over the shoulders and right underneath between the back legs.