The new Senate election system allows for political families to manipulate the results, according to Internet Dialogue on Law Reform (iLaw) manager Yingcheep Atchanont.
Writing on Facebook on Friday, he said he was mistaken when he thought previously that political dynasties known as "Ban Yai" (Big House) could not intervene in the process.
At first, he thought they could only manipulate elections at the district level by hiring people to apply as candidates to vote for other "preferred" candidates to progress. t
But many districts are not competitive, Mr Yingcheep said. Some small districts only have a few candidates, meaning they can all ascend to the provincial-level vote.
The election comprises three levels of voting at district, provincial and national levels.
Each level has intra- and inter-group voting among 20 professional groups until the final round when 10 people with the top votes from each professional group are selected to fill the 200 senator vacancies.
Mr Yingcheep said those political families sent their representatives to contest in Muang district and sent other people to register in different districts in almost every professional group.
At least 15 people in each district passed that vote effortlessly. By using this strategy, they had at least 40 candidates in the provincial-level vote and could advance to the national vote, he said.
"There are many ban yai in Thai politics. They can form an alliance and even coerce independent candidates -- only a handful of whom can advance to the national level -- to join them to ensure their candidates secure seats in the final round of the election," Mr Yingcheep said.
"As the number of independent candidates is too low, it is not difficult for the big houses to manipulate the outcome," he said.
Meanwhile, the Election Commission (EC) has issued a document concerning procedures for the national-level Senate election due to take place at Impact Forum, a convention centre in Pak Kret district in Nonthaburi, on Wednesday.
Each candidate must arrive and register at the election venue by 9am. Any late entries will be disqualified.
In the first round of intra-voting, each candidate can vote for up to 10 individuals within their professional group. The top 40 in each group will proceed on to an inter-group vote.