Often, an ISP's upload speed will not be as clearly advertised, and may be one-tenth of the headline download speed. Moreover, even on NBN plans advertising fast download speeds, the upload speeds often have the same lower bound, typically 1Mbps: iiNet's summary, page-2.
Applications like Zoom demand a minimal upload bandwidth to support your outgoing video-feed. Zoom recommends at least 600Kbps for 1:1 video calls, at least 800Kbps for group video calls, and double these figures if using a (quite standard) 720p HD webcam.
With so many people working from home, these minimal upload bandwidths are barely attainable. The following figures (measured using speedtest.net) show the measured download and upload bandwidths from an NBN plan advertised as 25/5 - up to 25Mbps download, and up to 5Mbps upload. Clearly, at the very times I wish to use Zoom to stream lectures and laboratory sessions, other professionals in my suburb are preferring NetFlix, and Zoom's minimal requirements are not being met.
See also: ABC News Coronavirus affecting internet speeds, as COVID-19 puts pressure on the network.