Context : The Red MSX Source ( RMS ) survey has identified a large sample of candidate massive young stellar objects ( MYSOs ) and ultra compact ( UC ) HII regions from a sample of \sim 2000 MSX and 2MASS colour selected sources .

Aims : To search for H _ { 2 } O masers towards a large sample of young high mass stars and to investigate the statistical correlation of H _ { 2 } O masers with the earliest stages of massive star formation .

Methods : We have used the Mopra Radio telescope to make position-switched observations towards \sim 500 UCHII regions and MYSOs candidates identified from the RMS survey and located between 190° < l < 30° .
These observations have a 4 \sigma sensitivity of \sim 1 Jy and a velocity resolution of \sim 0.4 km s ^ { -1 } .

Results : We have detected 163 H _ { 2 } O masers , approximately 75 % of which were previously unknown .
Comparing the maser velocities with the velocities of the RMS sources , determined from ^ { 13 } CO observations , we have identified 135 RMS-H _ { 2 } O maser associations , which corresponds to a detection rate of \sim 27 % .
Taking into account the differences in sensitivity and source selection we find our detection rate is in general agreement with previously reported surveys .

Conclusions : We find similar detection rates for UCHII regions and MYSOs candidates , suggesting that the conditions needed for maser activity are equally likely in these two stages of the star formation process .
Looking at the detection rate as a function of distance from the Galactic centre we find it significantly enhanced within the solar circle , peaking at \sim 37 % between 6–7 kpc , which is consistent with previous surveys of UC HII regions , possibly indicating the presence of a high proportion of more luminous YSOs and HII regions .