There are various emergency provisions in strata law but the simplest thing might be for you to arrange an Extraordinary General Meeting (under Section 19, below) where you can present your plans and get them passed.
There shouldn’t be any objection to either the meeting or the proposal as you are taking away the owners corporation’s responsibility to fix the bathroom waterproofing.
Normally, you would have to bear the cost of the EGM and the by-laws written for it (unless there is a template in your existing by-laws) but since this is to everyone’s benefit, you might reasonably ask the owners corp to chip in.
All things being equal, the secretary will call the EGM, very few people will turn up, someone will object but they will be outvoted and you can get on with your renovation.
Be as accommodation as you can on issues like noise, removal of rubbish and use of qualified, licensed tradies and give your neighbours no reason to object.
But if they do, you can seek orders at NCAT forcing them to fix the leaks at their expense as a matter of urgency (and usually explaining that pulls the recalcitrants into line).
Regarding disruption to tenants, consider offering a rent reduction and minimising the time they will be unable to use the bathroom.
To calculate that, take the estimate of the time that bathroom installers have give you, add 50 percent and then another week, because that’s how long these things take in the real world.
Or put a penalty clause in the contract so that they pay for any extra time the bathroom is out of commission.
19 Other general meetings
(1) The secretary or a strata committee of an owners corporation may convene a general meeting (that is not an annual general meeting) of the owners corporation at any time.
(2) The secretary of the owners corporation, or another officer if the secretary is absent, must convene a general meeting (that is not an annual general meeting) of the owners corporation as soon as practicable, and not later than 14 days after, receiving a qualified request.
(3) A meeting may be convened on a qualified request even if the first annual general meeting has not been held.
(4) A request is a qualified request for the purposes of this section if it is made by one or more owners of a lot or lots in the strata scheme having a total unit entitlement of at least one-quarter of the aggregate unit entitlements.