In my experience white flies are relatively hard to completely eliminate, I find most insects tend to be nesters but whitefly have large populations that migrate a lot and if you are in an unlucky location it doesn't matter what kind of pesticide regime you use there will always be some population of whitefly that passes over your plants for a quick feed regularly.
My best advice in regards to white fly is to mow your lawn often, they seem to enjoy long grass and their populations will build up if you don't maintain your yard. I have found a systemic organic insecticide like neem oil or its extracts (I tend to favour an extract called Eco-Neem) can be quite effective as a deterrent, both when sprayed and used as a soil drench. Insecticidal soap is also a good insecticide I have used to control insect numbers as well and is not harsh on your plants when used in a reasonable quantity, which is more than I can say for most pyrethrum formulations and heavy doses of spinosad.
Despite the fact white fly seem to be a relatively migratory plant feeder you do need to consider the fact that they will lay eggs on the underside of your leaves and their offspring will worsen your problem.
Insects weaken the plant, which makes in turn makes it more susceptible to further insect infestation, white fly are an easy pest to identify due to their relatively large size, distinct appearance and the fact that they tend to buzz around your plant making their presence obvious. There may be several varieties of insects that are now having a feast on your plants, don't assume the damage is just from whitefly, check if there are any other kind of bugs on your plants and if there are take immediate action to control them.
Sticky traps for flying insects placed around your plants, regular application of neem oil or it's extracts and some insecticidal soap should help control insect populations fairly effectively, I would urge you to get on top of this problem now because you really want to be applying the minimal amount of insecticide to your plants once they begin flowering, which is likely to be anywhere from 2-4 weeks from now. You can probably get away with using soil drenches with neem or its extracts for 3 or 4 weeks into flower but even that might be pushing it.
When I was in late flower and still had an insect problem I started spraying insecticidal soap on my hands and then wiping it on the leaves, this way I managed to avoid getting any soap on the buds but it still helped control the problem.
I think they will flower soon because I have never seen a cannabis plant in mid to late February that wasn't flowering, cannabis flowers based on photoperiod and by that time of year the days are short enough that cannabis will flower unless it is not mature enough to, from memory a plant only needs 4-6 weeks before it is sexually mature which means unless you are planting in January your cannabis plant is highly likely to be flowering by Feb.
I don't know what April/May is like in QLD, but it can get pretty wet down here in Victoria and you have to be mindful of that, unless you have a greenhouse you really want your plants flowering in Feb to finish in March/April because you are likely to see considerable losses to mould due to rain and high humidity if your plants are finishing in May.
Ive used premium organic potting mix and half strength seasol so it cant be a deficiency 
