New surveys such as ESAs Euclid mission and NASAs Nancy Grace Roman Space Telescope are planned to map with unprecedented precision the large-scale structure of the Universe by measuring the 3D positions of tens of millions of galaxies. It is necessary to develop theoretically modelled galaxy catalogues to estimate the expected performance and to optimise the analysis strategy of these surveys. We populate two pairs of 1 (Gpc/h)$^3$ volume dark-matter-only simulations from the UNIT project with galaxies using the SAGE semi-analytic model of galaxy formation, coupled to the photoionisation model GET_EMLINES to estimate their Halpha emission. These catalogues represent a unique suite that includes galaxy formation physics and - thanks to the fixed-pair technique used - an effective volume of ~ (5 Gpc/h)$^3$, which is several times larger than the Euclid survey. We study the abundance and clustering of those model star-forming Halpha emission-line galaxies (ELGs). For scales greater than ~5 Mpc/h, we find for the ELGs a scale-independent bias with values in the range b $in$ [1,4.5], increasing with redshift over the interval z $in$ [0.5,2]. Model galaxy properties, including their emission-line fluxes are publicly available.
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