Merdeka Center attributes the decline to increased concerns over the spiraling cost of living as consumers begin to feel the impact of recent increase in fuel and electricity prices.
The pollster also acknowledged that the way the government handled the Bersih 2.0 rally had generated some "adverse" negative perceptions and eroded the prime minister's support.
The latest announcement puts the prime minister's approval rating at its lowest point in nearly two years.
Najib's approval rating has been on a steady decline since June 2010, after having achieved a record high of 72 percent.
The survey period was between Aug 11 and Aug 27 which coincided with the announcement of the proposed parliamentary select committee on electoral reform, and a month after the Bersih rally of July 9.
Along ethnic lines, Najib's support among ethnic Malays declined slightly by four points, down from 73 percent, whereas support from the ethnic Chinese tumbled, dropping a whopping 11 points to 38 percent.
The outlook of Indian Malaysians on the direction the country has taken also took a dive from 54 to 39 points, but interestingly, they are the only group to have increased their support for Najib, up two points from 67 percent.
Based on the survey, almost one third of respondents are worried about the rising cost of living, a concern that cuts across ethnic lines.
Najib had on July 27 announced that the seventh National Key Results Area will be introduced to the Government Transformation Programme to address the spiralling cost of living, but it appears to have done little to soothe concerns.
May - 45%
June - 65%
July - 64%
Sept - 56%
Dec - 66%
March - 69%
May - 72%
Nov - 69%
March - 67%
May - 65%
Aug - 59%