  The Murchison Widefield Array (MWA) is an electronically steered low
frequency ($<300$\,MHz) radio interferometer, with a `slew' time less than
8seconds. Low frequency ($\sim 100$\,MHz) radio telescopes are ideally suited
for rapid-response follow-up of transients due to their large field of view,
the inverted spectrum of coherent emission, and the fact that the dispersion
delay between a 1GHz and 100MHz pulse is on the order of $1-10$\,min for
dispersion measures of $100-2000$\,pc/cm$^3$. The MWA has previously been used
to provide fast follow up for transient events including gamma-ray bursts, fast
radio bursts, and gravitational waves, using systems that respond to gamma-ray
coordinates network (GCN) packet-based notifications. We describe a system for
automatically triggering MWA observations of such events, based on VOEvent
triggers, which is more flexible, capable, and accurate than previous systems.
The system can respond to external multi-messenger triggers, which makes it
well-suited to searching for prompt coherent radio emission from gamma-ray
bursts, the study of fast radio bursts and gravitational waves, single pulse
studies of pulsars, and rapid follow-up of high-energy superflares from flare
stars. The new triggering system has the capability to trigger observations in
both the regular correlator mode (limited to $\geq 0.5$\,s integrations) or
using the Voltage Capture System (VCS, $0.1$\,ms integration) of the MWA, and
represents a new mode of operation for the MWA. The upgraded standard
correlator triggering capability has been in use since MWA observing semester
2018B (July-Dec 2018), and the VCS and buffered mode triggers will become
available for observing in a future semester.
