We generalise a result from [Bak15] on K3 type hyperk\"ahler manifolds proving that a line in a lagrangian plane on a hyperk\"ahler manifold $X$ of Kummer type has Beauville-Bogomolov-Fujiki square $-\frac{n+1}{2}$ and order 2 in the discriminant group of $H^2(X,\mathbb{Z})$. Vice versa, an extremal primitive ray of the Mori cone verifying these conditions is in fact the class of a line in some lagrangian plane. In doing so, we show, on moduli spaces of Bridgeland stable objects on an abelian surface, that lagrangian planes on the fibre of the Albanese map correspond to sublattices of the Mukai lattice verifying some numerical condition.