At critical sites, most generators are fired up monthly for at least a 15 minutes. Most companies hate doing that because of the cost but do it to keep their tier 4 rating.
I think all of NextDC are tier4 which means they have redundancies and fault tolerance to keep 99.999% uptime (about 5 minute outage per year).
I don't understand all the NextDC hate though. They are the Australian DC company that just supplies each space to other businesses and government agencies. NextDC don't runs servers that mine your data or generate shitty videos, they supply space to for businesses to run servers. Go hate on Microsoft or Amazon for their massive datacentres that not only kill the environment but sell out our privacy as well.