The statements and images from PLAN are indeed useful. 4km (as confirmed by both Chinese and Australian sources) does not strike me as a particularly close approach. The P-8's deployment of one or more sonobuoys is much more interesting, for the sonobuoy must be intended to gather data concerning the signature of either the PLAN surface vessels, or a known or suspected PLAN submarine. In either case, particularly the latter, it does not seem unreasonable to regard this as a mildly hostile act deserving of a response in kind. The use of a sonobuoy is curious for another reason also: namely that the PLAN surface vessels could potentially recover it, something of which the personnel aboard the P-8 and those authorising its deployment must surely have been aware.
The other item of interest is that it has not, to my knowledge, been alleged that the PLAN laser was targeted at the cockpit of the P-8 in question. That is to say, the Australian account is consistent with PLAN's account of use of a laser ragefinder as a rangefinder, i.e. not as a blinding weapon.
At this point my interpretation of events is that we are seeing is a fairly unremarkable example of military-military communication between hostile powers, in which displays of one kind (close surveillance, dropping sonobuoys) are met with displays of another kind (laser rangefinder). Given the Australian political climate over the past few weeks, with an election imminent and a government that has been trying to portray itself as "tough on China" while portraying the opposition as "weak on China", it seems likely that this encounter was either explicitly orchestrated by the Australian government, hoping to elicit a response that it could then employ for domestical political purposes or, and I think this is more likely, rather occurred "organically", and was then subsequently seized upon and sensationalised by the government for those same purposes.