First of all, you should make a new rectangle with your bounds:
var southWest = new google.maps.LatLng(-31.203405,125.244141);
var northEast = new google.maps.LatLng(-25.363882,131.044922);
var loc = new google.maps.LatLngBounds(southWest,northEast);
var rectangle = new google.maps.Rectangle({
bounds: loc,
editable: false,
draggable: false,
strokeColor: '#FF0000',
strokeOpacity: 0.8,
strokeWeight: 2,
fillColor: '#FF0000',
fillOpacity: 0.35
});
rectangle.setMap(map);
And then just use an event listener on that rectangle
google.maps.event.addListener(rectangle, 'mouseover', openInfoWindow);
function openInfoWindow(event) {
var ne = rectangle.getBounds().getNorthEast();
var sw = rectangle.getBounds().getSouthWest();
var content = 'Infowindow content';
// Set the info window's content and position.
infoWindow.setContent(contentString);
infoWindow.setPosition(ne);
infoWindow.open(map);
}