Context. The extrasolar planet WASP-67 b is the first hot Jupiter definitively known to undergo only partial eclipses. The lack of the second and third contact points in this planetary system makes it difficult to obtain accurate measurements of its physical parameters.

Aims: By using new high-precision photometric data, we confirm that WASP-67 b shows grazing eclipses and compute accurate estimates of the physical properties of the planet and its parent star.

Methods: We present high-quality, multi-colour, broad-band photometric observations comprising five light curves covering two transit events, obtained using two medium-class telescopes and the telescope-defocusing technique. One transit was observed through a Bessel-R filter and the other simultaneously through filters similar to Sloan g'r'i'z'. We modelled these data using jktebop. The physical parameters of the system were obtained from the analysis of these light curves and from published spectroscopic measurements.

Results: All five of our light curves satisfy the criterion for being grazing eclipses. We revise the physical parameters of the whole WASP-67 system and, in particular, significantly improve the measurements of the planet's radius (R_b = 1.091 ± 0.046 R_Jup) and density (rho_b = 0.292 ± 0.036 rho_Jup), as compared to the values in the discovery paper (R_b = 1.4 _-0.2^+0.3 R_Jup and rho_b = 0.16 ± 0.08 rho_Jup). The transit ephemeris was also substantially refined. We investigated the variation of the planet's radius as a function of the wavelength, using the simultaneous multi-band data, finding that our measurements are consistent with a flat spectrum to within the experimental uncertainties.