Family First has been excluded from running at the Kiama by-election.
The NSW Electoral Commission this week advised the party it could not accept candidate Kyle Napoleoni's nomination.
Family First was registered in April but little known state government legislation forbids a party from running for 12 months after it achieves registration.
National Director and NSW Upper House candidate Lyle Shelton said the party was bitterly disappointed.
"These petty rules, which only exist in NSW, seem designed by the major parties to make it hard for smaller parties.
"When it comes to election rules, the fix is in by NSW Liberal and Labor. Nothing unites the major parties more than keeping the competition out.
Mr Shelton said Family First was disappointed it could not offer the people of Kiama third party insurance at the September 13 by-election but vowed the party would be back at the general election.
"I'd like to thank Kyle and his family for being willing to step forward and offer the people of Kiama a family-friendly choice.
"Family First is working to field candidates in all 93 lower house seats in the lead-up to the March 2027 state election," Mr Shelton said.
