Ms Williams spent a month leading up to Anzac Day planning the cake, which has a fondant rifle, poppies, boots, and an Anzac medal on it.
Everything on the cake is edible except for the wires holding up the poppies.
A Facebook post showing the cake has received more than 12,000 shares online.
"A couple of weeks before Anzac Day, I kicked into action and just started researching what was out there to start with, because I didn't want to take the beauty away from other cake-makers," she said.
Ms Williams used to be in the New Zealand Army as a caterer.
She had two grandfathers who fought for New Zealand and said she wanted to "honour the RSA because they are servicemen, not only past but present".
She did not tell the RSA she was making the cake until a week before Anzac Day.
"It was my gift," she said.
Ms Williams' research included a trip to the Army base in Whanganui.
"It was wonderful. It was a wonderful learning experience."
When she presented the cake to the RSA for Anzac Day, she said it was "overwhelming".
Ms Williams was able to cut and serve the cake herself.
"One guy was explaining to me about the rifle and he was quite proud of me that I got it right."
Ms Williams, who has made themed cakes in the past, believes there is a trend towards detailed cakes.
She would call herself an artist first and a baker second.
Ms Williams has had some "really interesting requests" for cakes.
"People aren't holding back," she said.
A Fifty Shades of Grey-themed cake Ms Williams made was a hit online recently.
"The artistry side of it is where I really blossom, and that's where my business really performs better. So when people are asking for a cake from me, it's actually artwork they're asking for. The cake is a bonus."
Wanganui RSA manager Kyle Dalton thought the Anzac cake was "absolutely fantastic".
"When everyone saw it they didn't want to cut it," he said.
Mr Dalton said the cake "actually tasted better than it looked, if you can believe that". "The medal disappeared, then the boots, and the rifle.
"We're extremely grateful ...
"We're just blown away by her generosity and the support of the club and that she thought of us on Anzac Day," he said.