The Nationals have expressed “total dissatisfaction” with Australia Post's delivery and service standards.

Just as the Government-owned business starts mailing out same-sex marriage survey forms, Steph Ryan, Deputy Leader of the Victorian Nationals, moved the motion at a national conference saying complaints against Australia Post had tripled since 2008.

The motion called on the “Federal Government to ensure an effective postal service model, and records our total dissatisfaction with the current level of service provided by Australia Post”.

Ms Ryan noted that the Commonwealth Ombudsman has found Australia Post has the second highest complaint rate of any entity.

The criticism came just as Australia Post started sending out survey forms to millions of Australians this week.

The Australian Bureau of Statistics, which is overseeing the survey, says possible delivery failures by Australia Post are a threat to the legitimacy of the process.

Deputy Prime Minister Barnaby Joyce says he is confident Australia Post is up to the job.