Groundwater dynamics at the hillslope–riparian interface in a year with extreme winter rainfall

•We monitored GW-SW dynamics along a hillslope transect during a >200 year flood.•Artesian GW behaviour in riparian zone and on upper hillslope.•Deeper GW response slower than fast responding shallower GW during events.•Isotopes reveal remarkably well-mixed GW system recharged by winter precipitation.