Context : Water fountain nebulae are AGB and post-AGB objects that exhibit high-velocity outflows traced by water maser emission .
Their study is important to understand the interaction between collimated jets and the circumstellar material that leads to the formation of bipolar/multi-polar morphologies in evolved stars .

Aims : To describe the three-dimensional morphology and kinematics of the molecular gas of the water-fountain nebula IRAS 16342 - 3814 .

Methods : Retrieving data from the ALMA archive to analyse it using a simple spatio-kinematical model .
Using the software SHAPE to construct a three-dimensional spatio-kinematical model of the molecular gas in IRAS 16342 - 3814 .
Reproducing the intensity distribution and position-velocity diagram of the CO emission from the ALMA observations to derive the morphology and velocity field of the gas .
Using CO ( J =1 \rightarrow 0 ) data to support the physical interpretation of the model .

Results : A spatio-kinematical model that includes a high-velocity collimated outflow embedded within material expanding at relatively lower velocity reproduces the images and position-velocity diagrams from the observations .
The derived morphology is in good agreement with previous results from IR and H _ { 2 } O maser emission observations .
The high-velocity collimated outflow exhibits deceleration across its length , while the velocity of the surrounding component increases with distance .
The morphology of the emitting region ; the velocity field and the mass of the gas as function of velocity are in excellent agreement with the properties predicted for a molecular outflow driven by a jet .
The timescale of the molecular outflow is estimated to be \sim 70-100 years .
The scalar momentum carried by the outflow is much larger than it can be provided by the radiation of the central star .
An oscillating pattern was found associated to the high-velocity collimated outflow .
The oscillation period of the pattern is T \approx 60-90 years and its opening angle is \theta _ { op } \approx 2 ^ { \circ } .

Conclusions : The CO ( J =3 \rightarrow 2 ) emission in IRAS 16342 - 3814 is interpreted in terms of a jet-driven molecular outflow expanding along an elongated region .
The position-velocity diagram and the mass spectrum reveal a feature due to entrained material that is associated to the driving jet .
This feature is not seen in other more evolved objects that exhibit more developed bipolar morphologies .
It is likely that the jet in those objects has already disappeared since it is expected to last only for a couple of hundred years .
This strengthens the idea that water fountain nebulae are undergoing a very short transition during which they develop the collimated outflows that shape the CSE .
The oscillating pattern seen in the CO high-velocity outflow is interpreted as due to precession with a relatively small opening angle .
The precession period is compatible with the period of the corkscrew pattern seen at IR wavelengths .
We propose that the high-velocity molecular outflow traces the underlying primary jet that produces such pattern .