The move comes in a jubilee year, but Girl Guides Australia director Belinda Allen says the timing is right.
"The Queen was contentious with a number of our members, not all our members by any means," she told Lateline.
"It was very much felt that she wasn't the word required, but people could very much serve her by serving Australia."
The move follows 18 months of consultation with thousands of members of Australia's largest volunteer girls group, most of them girls between the ages of 10 and 14.
Ms Allen says the decision is controversial among older members in the group's 800 districts.
"It's been a huge, huge decision and it hasn't been taken lightly at all," she said.
"We very, very much respect for many of our members that's a promise that's dear to their heart and that's a promise they made as a young women and they have lived by all their life."
The removal of Her Majesty will be optional, and as a residual mark of respect the name of the guide's highest honour - the Queen's Guide - will not change.
Guide leader and Queen's Guide Gillian Garsia agrees it is time to modernise, but admits it is a bit of wrench.
"We've always loved the Queen in my home and watch all the celebrations," she said.
"[We have] recently been glued to all the jubilee things. I see her as our head of state."
In the new promise, duty to God is also gone.
"Our members didn't feel 'duty to God' reflected all faiths and belief systems across the world," Ms Allen said.