I am the Australian.

I read the information given in Form 888. This is what it says:

If you are outside Australia and are unable to get an Australian
citizen or Australian permanent resident to complete a
statutory declaration in support of your relationship with your
partner, you may obtain statements from people who know
you and your sponsor. Such statements are not statutory
declarations under Australian law. However, in accordance
with policy, they should be witnessed or certified according to
the legal practices of the relevant country. Failing that, they
should be witnessed by a person whose occupation or
qualification is comparable to those listed above. This person
should sign, date and specify their occupation at the bottom
of the statement.

The question that is worrying me now is; should the statements given by non-Australians be written in Form 888 or can they be written on plain paper and certified according to the legal practice in my partners country?