Auckland sits on volcanic field that could reactivate at any time and the best way to monitor volcanoes is to check for seismic signals.

With the screening of the 90-minute docu-drama Eruption on TV3 on Wednesday night, Aucklanders might wonder about the state of volcano monitoring in their region.

The portrayal in Eruption of a magma hot-spot under Auckland showing signs of unrest leading to an eruption is realistic and scientifically credible, according to volcanologists.

They expect any future volcanic activity in Auckland will be preceded by a warning period of precursory earthquakes that may last for a few days to a few weeks.

As the magma and gas force their way to the surface through cracks and other weak subterranean structures, vibrations and small earthquakes occur.

Listening for such telltale signals are nine permanent seismic instruments positioned around the Auckland region. The instruments are operated by the GeoNet project funded by the Earthquake Commission, and the real-time data they produce is monitored by GNS Science.

The original network of instruments, established in the 1980s, was recently upgraded by GNS Science's GeoNet project. It now provides more data and higher quality data and this equates to an improved standard of monitoring for the Auckland region.

Four of the nine instruments are installed in deep boreholes - between 300m and 400m deep - to get away from surface-generated noise such as traffic and construction. This provides a reliable and clear signal of any seismic activity occurring under Auckland.

Volcano Surveillance Co-ordinator at GNS Science, Brad Scott, said the new GeoNet monitoring equipment had been specifically designed to see volcanic unrest developing in Auckland and would provide better information for the responding agencies.

"It will also help to remove some of the uncertainty that goes with volcanic unrest."

The Auckland Volcanic Field covers 360sq km and stretches from Manurewa in the south to Takapuna in the north.

By monitoring the earthquake activity in the Auckland Volcanic Field, scientists and civil defence officials will be able to provide early warning of an impending volcanic eruption.

The location and strength of the signal will enable officials and emergency services to decide where to focus their response efforts.

The higher quality data being recorded by the upgraded Auckland network provides volcanologists with more information and more certainty on any earthquake activity in the region.

Normal 'background' earthquake activity in Auckland is very low compared to other parts on New Zealand. Typically, there might be a handful of very small earthquakes each year, usually too small to be felt. So any heightened earthquake activity will be conspicuous.

Should volcanic unrest be detected in Auckland the Volcanic Alert Level would be raised by GNS Science to reflect the level of unrest. This five-stage mechanism is used to set responses by responding agencies.