Appears works are starting on the new substation that will be...

Appears works are starting on the new substation that will be required for NextDC S2 and Macquarie Telecom to achieve full power rating. 

I'd estimate ~24 months before either could take supply from it.

Assuming they have negotiated a reasonable interim limit, I don't think the timeline should be a bottleneck. 

There are practical limits to how quickly they can build out internal infrastructure, fit out data halls and then actually have customers moved in.

They appear to be staging the initial Cooling infrastructure at Wicks & Waterloo, so you could maybe use that to confirm the initial buildout capacity.