From what I can determine, the last time ABC/774/3LO rated below 5% was in Survey 1/1963 when it rated 2.6%. This was just before ABC did a revamp of its two radio networks and flipped program formats between them, bringing light entertainment and general programming back to 3LO, and 3AR becoming the fine music/specialist talk station. (Although 3LO would still have its schedule interrupted by large slabs of parliament broadcasts for the next 25 years)
3AR 6.3
3LO 2.6
3UZ 28.1
3DB 22.2
3KZ 12.3
3AW 15.6
3XY 5.5
3AK 6.3
Other 0.8
In the next survey, 3LO rated 5.9%
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