Twigg, from Hawkes Bay, tweeted: "Thanks EuroCity and Skoda NZ for the new wheels, supporting HB talent and driving me towards London2012."
Shanks posted: "I got given a BRAND NEW Mazda 3 from Dunedin City Mazda - Zoom Zoom Zoom. Hot! Photo coming .... if you haven't already seen it around town."
Other Kiwi hopefuls have heavily mentioned everything from "delicious" local beef and lamb to sunblock products and sports equipment on their Twitter accounts in recent weeks.
Some of the home-grown promotions are coming close to strict rules surrounding sponsorship, according to Dave Currie, New Zealand's chef de mission for the Olympics.
"We have spent a lot of time in the past three months having meetings with the athletes and have gone to great pains to make it clear what the sponsorship regulations are," he said.
"With regards to our people plugging cars, the Skoda mention [from Twigg] in particular is a tad marginal.
"In the run-up to the Games it is okay to mention you are being sponsored by a local firm, but you are not allowed to make a link back to the Olympics."
Currie said athletes were also not allowed to push products in direct competition with any official sponsors of the national team.
He added that when the Games began on July 27, and for about two weeks afterwards, athletes from all countries were banned from mentioning sponsors other than those officially endorsing the Games.
"This is the first Olympics where social media will really come to the fore and it is a whole new challenge for everyone about how it will be managed," Currie said.
"Aside from refraining from advertising, our general advice to our athletes is to engage the brain before hitting the send button."
Twigg told the Herald on Sunday she didn't believe she did anything wrong by tweeting about her new Skoda, given to her for six months by the Napier branch of EuroCity.
She also insisted she only mentioned a Kiwi sunblock firm a few times recently because it was local.
"Nothing on my vehicle suggests anything about the Olympics and I only mention firms from the Hawkes Bay area," Twigg said.
"For me, the Olympics is not about making a lot of money. If that were the case I'd probably be doing something other than rowing," she said.
"It is not exactly a lucrative sport."