Senior Constable David Cogger said police search and rescue and Coastguard were called by the girls' male friend, who had been able to swim back to the beach.
"At near to low water last night the three teenagers stripped off and waded out on the sand bar near the blind channel at the western end of Tahunanui Beach.
"Due to the urgency of the situation a shoreline search by a police dog handler and a response staff member located the two young women."
He said police officers entered the water and rescued them, before they were taken to Nelson Hospital for observation and treatment.
Mr Cogger issued a warning for other people contemplating a late-night swim at the beach.
"The western end of Tahunanui Beach can be full of holes and deep water pockets and is not a safe place to go for a midnight swim. The Blind Channel in certain tides and wind conditions is a very dangerous place to be even in daylight."
The beach rescue came after a group of four trampers also had to be rescued near Takaka.
Mr Cogger said the group "strayed off" the Boulder Lake Track and became lost.
The group had five days' food and a locator beacon and were heading into the park on day one of their tramp when they got lost.
"It took quite some considerable time to actually work out where they were and it took two attempts to direct them back to the track.
"Department of Conservation and Landsar Golden Bay assisted police search and rescue to direct the group back to the track.
"Modern technology is a great aid to those venturing into the outdoors, however a good current map and compass is still an essential part of every party's gear list. GPSes and smartphones need to be calibrated on the correct mapping datum and the user must be familiar with its functions."
The group was finally directed back on track about 7.45am today.
