Tygothane. Polyurethane tubing, Formulation C-210A. Here in the states I get mine from McMaster Carr. Down under I have no idea who carries it. Search the formulation and make sure it is the correct one. Other polyurethane formulations don't work......

No need to keep any fuel in the tank to keep this formulation supple. My original experimental tank is still in service 3 years and running, without any issues. I use the 1/8" x 3/16" size and it is the most flexible stuff I've found (including viton in 1/8x1/4 size). It is about 1/10 the cost of viton but that's minor issue since one only uses a couple feet to plumb the gas supply. You have two viable options