Alzheimers Tauranga board member Libby Hewitt knew Mrs Hallam before she joined the society, due to her father having Alzheimer's.
"She's just a lovely, lovely woman," she said.
Ms Hewitt said the celebration commemorated the hard work she had put into the society.
"A lot of people have been invited whose lives she's touched," Ms Hewitt said.
"We are a very heartfelt organisation. We're all connected to this disease."
MP Simon Bridges was among the guests, as was the current chairwoman of the Tauranga Alzheimer's Society Board Kathy Webb, and also the previous chairman David Ansell.
Part of Mrs Hallam's contribution to the Alzheimer's Society was to establish a group called The Young Ones, also known as "The Fun Ones", short for Forget Me Not.
The group was created after Mrs Hallam saw a need for a support group for partners and carers of younger people with dementia.
Maire Pierson's husband first showed symptoms at only age 56.
"Things weren't good," Ms Pierson said.
Despite having a "beautiful marriage", the stress of the disease was taking its toll on both Ms Pierson and her husband.
After a heated argument, or "ding dong go" as Ms Pierson put it, her husband picked up the phone and called the Alzheimer's Society, saying they needed help.
"Valma answered, and she came that same day," Ms Pierson said.
The group would get together, have pot luck dinners, and talk, laugh and share their experiences.
Sue Fursdon of Bellevue joined the group later in 2006 after her husband was diagnosed.
"It was just a whole new world. A real learning curve," she said.
"Valma was just wonderful. She's been our rock."
Ms Hallam spoke at the celebration, and thanked everyone who had helped her and had contributed to the Queen's Service Medal.
"I stand here today humbled, and feel okay, or dare I say good, about what has been achieved within the team of so many people whose desire it is to love and serve their fellow human beings," Mrs Hallam said.
Dementia:
There has not been a single factor found to be the cause of dementia and there is not yet a cure. However, there are lifestyle changes which can reduce the disease.
* Look after your heart
* Be physically active
* Follow a healthy diet
* Challenge your brain
* Enjoy social activities
Source: alzheimers.org.nz