A tourist has been flown to Invercargill Hospital after she fell 20 metres down a waterfall while tramping on the Milford Track.

The 25-year-old Israeli tramper was spotted by a helicopter just before 10am Monday.

Te Anau police say the woman fell down the waterfall in the Mackinnon Pass and suffered a broken leg, heavy bruising and lost teeth.

They say she is in a serious condition.

The woman was expected to finish her tramp at Sandfly Point near Milford Sound early Sunday afternoon but failed to arrive.

Police say it is doubtful whether she would have survived another night due to the heavy wind and rain that has buffeted the area.