  We prove that the structure of Lusztig's semi-infinite Deligne--Lusztig
variety for $\mathrm{GSp}$ (and its inner form) is isomorphic to an affine
Deligne--Lusztig variety at infinite level, as a generalization of
Chan--Ivanov's result. Furthermore, we show that a component of some affine
Deligne--Lusztig variety for $\mathrm{GSp}$ can be written as a direct product
of a classical Deligne--Lusztig variety and an affine space, up to perfection.
We also study varieties $X_{r}$ defined by Chan and Ivanov, and we show that
$X_{r}$ at infinite level can be regarded as a component of semi-infinite
Deligne--Lusztig varieties even in the $\mathrm{GSp}$ case. This result
interprets previous studies on representations coming from $X_{r}$ as a
realization of Lusztig's conjecture.
