Jelzee

Im Australian and my partner is English. We've applied for a partner visa and been told we need more proof that we were in a defacto relationship before we applied. We were living with his dad and his dad has done a stat dec, plus we've got letters and bills etc addressed to us there but they said they need a lease or rental agreement. We didnt have one because it was his dads house but now were trying to work out if theres a way to make one and back date it? If not is there anything else we can provide to prove we were living in a defacto relationship? Thanks