The new compound Li3Mg2RuO6 exhibits interesting structural and magnetic properties. The Ru5+ sublattice is intrinsically frustrated : chains, along various [110] directions, of edge-sharing triangles linked by corners along [001]. According to bulk measurements, and preliminary neutron diffraction data (structure not resolved), the system orders antiferromagnetically at 17K and has a large magnetic frustration index of 6.14. Spin-dimer analysis indicates that in-plane 2nd and 3rd neighbour interactions are much stronger than both out-of-plane 1st and 4th neighbour interactions. This suggests that the magnetic frustration is essentially 2D which is not consistent with the fairly symmetric diffuse scattering observed just above TN. Here we propose to investigate the nature of the spin dynamics in Li3Mg2RuO6 using muon spin relaxation and obtain information about the magnetic interactions.