How much warning do you get when a tsunami happens?

How do you know when a tsunami occurs? How does the warning system work? When is a warning issued?

Following devastating Pacific-wide tsunamis in 1946 and 1960, a Pacific tsunami warning system was set up in Hawaii. Now similar warning systems operate in Alaska, the Caribbean and the Indian Ocean.

"There are a whole bunch of monitors around the planet that are set up to record earthquakes and if these are large enough, they may generate tsunamis", explains Professor James Goff, director of tsunami science at the University of New South Wales.

"When any event like this happens these sensors all around the world instantaneously pick up a signal".

Read more...http://www.abc.net.au/science/articles/2014/12/17/4027721.htm