Colimits of internal categories

We show that for an extensive $1$-category $\mathcal{E}$ with pullbacks and pullback stable coequalisers in which the forgetful functor $\mathcal{U}:\mathbf{C}\mathbf{a}\mathbf{t}(\mathcal{E}{)}_1\to \mathbf{G}\mathbf{p}\mathbf{h}(\mathcal{E})$ has left adjoint, the $2$-category $\mathbf{C}\mathbf{a}\mathbf{t}(\mathcal{E})$ of internal categories, functors and natural transformations has finite $2$-colimits. In addition, $\mathbf{C}\mathbf{a}\mathbf{t}(\mathcal{E})$ is extensive, has pullbacks and codescent coequalisers are stable under pullback along discrete Conduché fibrations. Moreover, we give converse results to this.