  We show that in wavepacket tunnelling localisation of the transmitted
particle amounts to a quantum measurement of the delay it experiences in the
barrier. With no external degree of freedom involved, the envelope of the
wavepacket plays the role of the initial pointer state. Under tunnelling
conditions such 'self measurement' is necessarily weak, and the Hartman effect
just reflects the general tendency of weak values to diverge, as post-selection
in the final state becomes improbable. We also demonstrate that it is a good
precision, or 'not really weak' quantum measurement: no matter how wide the
barrier d, it is possible to transmit a wavepacket with a width {\sigma} small
compared to the observed advancement. As is the case with all weak
measurements, the probability of transmission rapidly decreases with the ratio
{\sigma}/d.
