Riders4Helmets co-founder Lyndsey White said more retailers in Australia and New Zealand were sought for the awareness day.
"Please consider participating in order for us to continue to educate equestrians globally on helmet safety and to fulfill our goal - to get more equestrians wearing helmets," she said.
It is anticipated that about 500 equestrian retailers from around the world will be involved in the day. Helmet brands already taking part in the promotion include Samshield, Troxel, Charles Owen, GPA, Aegis (Devon-Aire), Pegasus, Tipperary, Ovation, IRH and KEP Italia.
"Additional manufacturers are expected to commit to participating and will be announced in due course," White said.
Individuals or organisations wishing to hold an event to recognise International Helmet Awareness Day are urged to contact Riders4helmets for support materials.
"You can participate and show your support just by wearing a helmet on June 11, no matter whether you are trail riding, showing or competing," White said.
The Riders4Helmets campaign is officially endorsed by USEF, USEA, USDF, USHJA, AETA, ARIA, EAF, CHA, PRO, EMSA and many leading equestrians.
It was founded in early 2010 after Olympic dressage rider Courtney King Dye was seriously injured in a riding accident. King Dye, who remained in a coma for a month following her accident, was not wearing a helmet. She is currently undergoing rehabilitation.
Jeri Bryant donated her helmet campaign t-shirts (featuring the slogan "Strap One On - Everyone's Doing it") to an eBay store set up to raise funds for King Dye, and a partnership was formed, resulting in the Riders4Helmets campaign.