electrical wiring – is it common property

Our strata building’s  main power board is old and not up to current standards and requires an urgent upgrade. The electrical wiring in all units have been inspected and an upgrade to the wiring is essential  as part of the mains board upgrade and for the safety of the building. The EC has received a quote for the job. Some units have previously been rewired and require minimum work. However most require major work, which will vary from unit to unit. Are we required to split the total cost by the  owners  strata levy  % or can each owner pay the cost for the rewiring of their own unit and then split the cost of the mains board and common area rewiring based on the owners strata levy %?

It seems very unfair if some owners have paid for new wiring or purchased a unit with new wiring and be expected to  pay a % of the rewiring of units belonging to owners who have not paid to rewire.