SYDNEY (Reuters) - Internet usage in highly connected Australia has fallen for the first time, signaling that the once booming online industry may finally have reached its plateau, a survey showed on Monday.

Market research firm MISC (Market Intelligence Strategy Center) said its Internet Uptake Index, which measures a range of Web use data, fell three percent in the three months to the end of June -- the first decline since the index was devised in 1999.

The data indicated that around 300,000 Australian Internet users abandoned the Web between April and June.

The fall in the index followed a sharp slowdown in usage growth in the previous quarter, a result the firm had originally dismissed as a possible one-off because of explosive, often double-digit, quarterly increases before.

"While the fall may not seem immediately significant, in the context of regular quarterly growth rates of 20 percent that have been shown by the index in the past, it is a serious and unprecedented correction," MISC said in a statement.

Australia, with just 20 million people, has one of the highest rates of Internet penetration in the world.

Broadbrand access has, however, been slow to take off because of limitations in the terrestrial cable network and technical difficulties with the roll-out of telecoms giant Telstra's ADSL broadband service.

MISC said Internet usage could pick up again once the next generation of technology -- namely 3G mobile phone connectivity -- becomes widely available. For now, the industry had to expect growth to plateau.

MISC said a number of factors may have contributed to the contraction in Internet use in the June quarter.

They included stricter rules on content, the demise of free music downloads at Napster, the collapse of a number of Internet Service Providers in Australia, some high profile online fraud and the problems with ADSL high-speed access.

Telstra, the dominant market player, has also capped downloads, said MISC.