It is in the partner migration booklet (page 28):
"Statements from certain persons usually barred from applying for a Partner visa
If you:
are in Australia; and
do not hold a substantive visa; and
have had a visa refused or cancelled since your last entry to Australia (other than visa cancellation or refusal on character grounds or a Partner visa refusal); and
wish to apply for a Partner visa; then
in order to make a valid Partner visa application, you must provide statutory declarations from 2 persons who:
are aged 18 years or over;
are Australian citizens or Australian permanent residents; and
can support the existence of your relationship with your partner.
These statutory declarations can be either completed by using either the:
blank statutory declaration template available from the Attorney-General’s Department website http://www.ag.gov.au or
form 888 Statutory declaration by a supporting witness relating to a partner visa application available from the department’s website http://www.immi.gov.au/allforms/pdf/888.pdf or from any office of the department or Australian mission.
The statutory declarations must have been made no more than 6 weeks prior to the day you lodge your application.
Note: If you provide these 2 completed statutory declarations, and they give sufficient evidence to support the existence, as well as the history, of your relationship with your partner, in most cases they will also be acceptable for the purposes of assessing the social context of your relationship (see page 27)."