Fire rating is a relatively simple job these days.Fire rated plaster board such as firecheck or fire stop are readily available,and should be fixed to the ceilings of the downstairs units.Look at the Boral website for various systems of fire rating using their board.You will also need to consider any other openings that go from one unit to another,they need to be filled up with mortar of other fir proof materials.PVC pipes that penetrate should have fire collars around them that collapse and squash the pipe in the event of a fire,preventing the fire from getting through.Fire mastic and fire pillows are available to seal up gaps and penetrations such as cables.You would need to check your state strata titling laws with regard to plumbing and electrical cables passing through one unit to get to another.You may find that they can only pass through common property to get to each unit,unless they are underground.If you have timber passing through walls,you MAY have to cut these either side of the wall to provide a rating.Having said that,timber does have a rating that you would have to check.You would need to check the roof space to make sure there are no gaps,and that dividing walls go right to the under side of the roofing.You may also need to cut roof battens as they pass over dividing walls.
Hope this helps.

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