A new formula connecting the elliptic $6j$-symbols and the fusion of the
vertex-face intertwining vectors is given. This is based on the identification
of the $k$ fusion intertwining vectors with the change of base matrix elements
from Sklyanin's standard base to Rosengren's natural base in the space of even
theta functions of order $2k$. The new formula allows us to derive various
properties of the elliptic $6j$-symbols, such as the addition formula, the
biorthogonality property, the fusion formula and the Yang-Baxter relation. We
also discuss a connection with the Sklyanin algebra based on the factorised
formula for the $L$-operator.