If there is no capacity and no POA, then what you describe is the process to appoint a public guardian, which usually takes months. If his GP is happy with your dad's capacity, or a geriatrician is able to formally assess, there is no need for state involvement. State involvement is only when there are no decisionmakers and a public advocate/guardian needs to be appointed.in WA legal capacity is decided by the state administration tribunal after reviewing a very detailed questionnaire conducted by a DR
Ideally dad would sign the order, but you can co-sign you so there is no question of the legality of the intervention order even if dad's capacity is brought under question.see your dads lawyer about taking one out if you are not able to yourself. You can do it on his behalf.
You might also get some help from the support organisations such as Carer's Australia, aged care advocacy groups as well?