NZ Ministry of Civil Defence & Emergency Management tweeted that it was "thinking of those of you who've just had a wee shake on Canterbury Anniversary Day. Hope everyone's doing OK".
Herald reader Shane Coxhill said it was enough to wake him up from his nap.
Geonet spokesman John Callan said Cantabrians are "seasoned earthquake observers and have become expert at estimating magnitudes", but every new quake of this size still comes as a surprise.
Nearly 4000 people have reported feeling the earthquake, including one report from near Hanmer Springs and a couple from Geraldine near Timaru.
Geonet seismologist Dr Matt Gerstenberger said compared to other earthquakes - like the 2011 quake in Christchurch - this shake is on the smaller scale, but 4.2 is still significant.
Gerstenberger said this afternoon's shake was part of ongoing aftershocks post-the 2011 Canterbury 6.3 earthquake.
He said it would be normal to have some 3 magnitude shakes fairly close to when the 4.2 struck, "but most of those would have come already".
Christchurch City Councillor Deon Swiggs said the tremor "rattled and shook the place around a bit", and asked others on Facebook if they'd felt it.
Twitter users have described it as a decent jolt.
