SBS has played down the impact of a government-led change in its advertising rules that would allow it to air more ads in prime time, saying it currently struggles with filling its five-minutes-per-hour allowance. But a Crikey investigation has found that, at least during prime time -- when SBS has its biggest audiences and thus the greatest potential to make money off more ads -- ad slots do not go begging. To make up a budget shortfall, the government wants SBS to be able to air more ads per hour in periods of high advertiser demand. The proposal has upset the commercial TV sector, which fears any extra advertising on SBS will come at the expense of their own ad revenues. But at Senate estimates last week, SBS managing director Michael Ebeid tried to play down their concerns, saying SBS, the fourth-rated TV network, already had problems filling all its ad inventory, so the revenue from extra ads would be minimal. "In television, if your inventory is -- as we have -- five minutes an hour, your fill rate can vary, obviously, during the day and night depending on demand. We are rarely ever full of our five minutes an hour, so our ability to just double that five minutes is unrealistic, particularly from day one," he said. But Crikey has seen advertising logs that suggest in prime time at least, SBS' fill rates are very healthy. The government's proposed legislation would allow SBS to average out its 120 minutes of allowable advertising a day over 24 hours, provided it never airs more than 10 minutes per hour. The purpose of the legislation, which Crikey understands will be tabled this month, is to allow SBS to put more ads to air in periods of advertiser demand, like prime time and during other periods of high audience share, and fewer ads during periods when fewer people are watching. The commercial television networks have released modelling that suggests SBS could take $200 million in advertising that would otherwise go to the commercial TV sector over five years, but SBS strongly disputes the figures, partly, as Ebeid claims, because the broadcaster says it's already struggling to fill its advertising quota. But lobby group Save Our SBS has revealed that on every night the group audited, SBS aired the maximum allowable advertising on SBS One, its main channel. Save Our SBS is opposed to more advertising on SBS. Save Our SBS volunteers have been conducting spot-checks of the advertising that airs on SBS for years, carefully recording what airs, in order to monitor both the total ad time and content. The resulting data doesn't offer a complete breakdown of all the advertising that appears on SBS, but the logs are highly accurate, logging what is aired down to the second. Over the six years the group has been conducting these checks, SBS One has never dipped below 30 minutes of ad time over six hours of programming aired during prime time. The logs also keep track of on-air promos and sponsor announcements, some of which do not count towards SBS' five minutes of allowable advertising, according to Section 45 of the SBS Act. In addition to its allowable billed advertising, SBS airs around four minutes of promotional material every hour. This suggests the total disruption to an hour of programming is around nine minutes -- changing the SBS Act would lift it to 14 minutes. On channels other than the main SBS One, it does not appear SBS regularly fills its five minutes of advertising over the hour. A fairly typical log of the advertising that aired on SBS 2 on the night of April 9, 2011, for example, showed the channel airing just 23 minutes and 45 seconds of advertising in Melbourne, with the five-minute ad cap only reached between 6.30pm and 7.30pm (during an airing of the Hairy Bikers' Food Tour of Britain). Similarly, during the day, SBS often didn't entirely fill its allowable advertising. During the six-hour period from noon to 6pm on Tuesday, April 26, 2011, for example, SBS only aired 21 minutes of advertising. The logs are not definitive, but they indicate that SBS One is not struggling to fill prime-time ad slots. If the legislation passes, viewers should expect the broadcaster to quickly fill more ads during the evening, as the demand from advertisers already exists.