AAY – I’m guessing that the now uncooperative neighbour was the dissenting vote (?), but provided your renovations and associated changes to the common property (sewerage) were consented to by way of a special resolution of the Owners Corporation (O/C) at a General Meeting in accordance with Sect 65A of the NSW Strata Schemes Management Act (SCMA), and that’s been minuted, then the O/C itself should be arranging access to your neighbour’s garage in order to facilitate those works.
So your O/C should, either by way of the Secretary of its Executive Committee or by an instruction to its Strata Manager, immediately write to your neighbour to arrange access to their garage within a short timeframe in order to by its agents (i.e. you / your plumber) “carry out work in accordance with Sect 65 of the SCMA”, and I’d suggest that you concurrently try to reason with them about the need for that access.
Let’s hope that a concurrent approach by the O/C and by you will see a sensible and timely outcome, but regrettably if access is still refused, as it couldn’t be claimed to be due to an emergency and as Tess observed (post 3) a Magistrate would likely refer the matter to the Dept of Fair Trading who administers the SCMA, the best course of action would be for you to then take the matter to mediation there, with the option of your O/C (not you individually) seeking Orders in the NSW Civil and Administrative Tribunal to enable access to your neighbour’s Lot under Sect 145 of the SCMA.