"[She] was swimming below the falls and was pulled under, and failed to resurface," a police spokesperson said.
It's understood she got caught in an eddy, where the river water recirculates.
Murchison Volunteer Fire Brigade deputy chief John Hebberd told the Herald it was the third drowning he had been called to at the dangerous waterfall.
Despite warning signs warning not to swim there, the falls could appear to be relatively innocuous and swimmers often ignored them.
"The policeman I was talking to said it was the fifth [drowning he'd attended there] in five years so it's a regular one a year," he said.
"As far as I know she just went in for a swim and it's a very dangerous place so it looks like the current has taken her under.
"It just keeps happening; the signs are there and everything but no-one seems to take any notice. The problem is it's got what looks like a still pool but it's got a current that just sucks them under."
The 10m Maruia Falls, on the Maruia River, is a "block" waterfall, where water descends from a relatively wide stream or river, according to the New Zealand Waterfalls website.
In 2011, 39-year-old Ian Oliver Randall died after slipping and falling down the waterfall after he and a friend jumped over a safety barrier to take a look. The friend grabbed hold of Randall's arm but couldn't hold on and he fell into the water below.
In 2014, Christchurch man Darryl Rolton, 54, died after jumping off the falls into a raging whirlpool to try to rescue his friend, Frank Otto. Otto survived.
The latest tragedy comes just two days after Water Safety New Zealand issued a warning for people to take extra care in the water this summer.