Many of them would be trivial to break in to, if I didn't care about leaving physical damage behind and the only ramification would be I'd be caught on film (which would hardly be a major issue for someone willing to think it through).
For all the talk about being extremely secure, many are basically if not completely (usually completely) unmanned after hours, many are in normal office buildings in and around the various CBDs and rely on little more than a swipe card preventing you selecting a specific floor from the lift and not having a hammer to break the invariably glass door past said lift (which may be tempered glass but it's still only a couple cm embedded into an aluminium frame, so bullet-proofing isn't going to help, here). There are a handful of higher tiered ones scattered around that do have (a single) security guard(s) after hours but I they're usually little more than a concierge.
As with all things in Australia, the vast majority of our datacentre physical security comes down to our national security policy of "it'll never happen, so why worry about it".
I have been in datacentres that house equipment belonging to a certain American company, that starts with "G" and ends with "oogle" and the only enhanced security they had was a yellow mesh around their racks, made out of the same stuff that fails to protect the doors and windows of residential houses from 12 years olds on a daily basis.
I've been in the supposedly "most secure, tier 3" commercial datacentre in the country and seen the perimeter fence and main access doors propped open by reels of cabling, because electricians doing onsite work didn't want to have to be buzzed in, constantly, while collecting stuff from their vans. I've even had an electrician who was testing onsite UPS hold doors to secure areas open for me, without asking me who I was or if I had access to them (without me even asking him to). Security in Australian datacentres is not quite where it should be.