Winnifred Harvey of Oamaru asks :-
In removing an attractive rock on dry mud I was amazed to see a small frog underneath it. There appeared to be no crevices by which it could get in or out. Why was it there and how did it survive?
Jennifer Gillette, a zoologist at the University of Canterbury, responded.
Your frog probably belonged to one of the species introduced to New Zealand from Australia, since none of New Zealand’s native frogs are now found in the South Island.
Like all amphibians, one of the biggest challenges that these frogs face is that of conserving water in their bodies. Protecting themselves from temperature extremes is also important. Finding a suitable refuge underground (or under a large rock) is one strategy that a frog can use. It is a lot easier to prevent desiccation when hiding underground, and being underground allows a frog to avoid temperature extremes. Frogs may hide in refugia either as part of a strategy over the course of a 24-hour period (for instance, hiding during the day and emerging at night to forage and breed) or as part of a seasonal strategy (for instance, hiding during extended droughts, or during times of subfreezing temperatures in winter). An added advantage of hiding during daylight is avoiding predators, since many predators search for prey by day.
Although some frogs can tolerate air temperatures at or just below freezing for short periods of time, seeking refuge underground would allow them to escape extended periods of sub-freezing temperatures. A frog’s metabolic rate depends both on how active it is and on ambient temperatures. A frog hiding in an underground refuge in winter will have a relatively low metabolic rate, and therefore will not need to eat much and may be able to survive extended periods without needing to ingest any food.
Finally, it may have looked to you like the frog was trapped under the rock. However, it clearly had some way of getting to that spot under the rock, so it would have been able to come out the same way.
Thus, the short answer to your question is that the frog you discovered had probably found what was to it a very suitable underground (underrock?) refuge, and was just waiting either till nightfall or until conditions improved a bit before returning to an active life above ground. If you’ve removed the rock, be assured that frogs are adaptable and it should have been able to find a new place to hide without much trouble.