"The largest one we've had is a 6.2 and we've had at lead three that are between 5 and 6 in magnitude, and a large number in the 4 to 5 range."
However, because the epicentre of the first quake was quite a way offshore, not all of the lesser aftershocks would be recorded.
Aftershocks could be expected for the next several days.
Dr Ristau said the quake occurred because of bending of the Pacific tectonic plate, which subducts under the Australian plate to the east of Gisborne.
"As the Pacific plate dives beneath the Australian plate it kind of bends and cracks, so it looks like this earthquake actually occurred within the Pacific plate as a result of that bending and cracking, rather than on the boundary between the two plates."
Meanwhile, WorkSafe New Zealand recommends that workplaces carry out safety checks before allowing people back to work.
"Safety of you and your work colleagues must be the number one priority if you're heading to work today in the affected areas," WorkSafe Chief Executive Gordon MacDonald said.
"Although there have been no reports of damage to buildings, the last thing any of us want is an injury because a few simple checks haven't been taken.
"Listen to what Civil Defence are advising and act accordingly. They're the experts in these situations.
"Make a careful external visual inspection of your work premises. If you see cracks or have the slightest doubt about the integrity of the building, get an expert report before you go in.
"If you decide it's safe to enter, be extra vigilant as the contents of the building may have shifted; material may have fallen; there could be hazards you would not normally expect like spilled liquids or damaged racking for stored goods.
"If you know there are chemicals or other dangerous material in your workplace, make sure you're protected with good safety gear and be very careful when you first go in.
"Stop, look, assess - and don't take any risks. There are professionals who can help you make your workplace safe.
- Gisborne Herald
