HONIARA, Solomon Islands (AP) _ An Australian navy demolition squad today blew up a huge cache of American and Japanese artillery shells left on the World War II Guadalcanal battleground and exposed by flooding during a recent typhoon.

The ammunition dump stretched from the former landing strip for U.S. fighter planes at Henderson field to Red Beach, the landing point for the first U.S. Marine division assault against Japanese land-based forces in the Solomon Islands in 1942.

''There are literally hundreds of shells,'' said Australian consulate officer John Given. ''One area covers about an acre. You can't walk without stepping on mortar rounds, 105mm shells and land mines.''

Given, reached by telephone from Sydney, Australia, said local residents had known for years about the buried ordnance.

''You could go out and pick shells out of the ground,'' he said.

But the size of the dump was not known until May 19, when Typhoon Namu battered the Pacific island chain for 17 hours leaving 90,000 people - one third of the population - homeless. More than 100 people died in the storm, the worst in the recorded history of the Solomon Islands.

''The dump is massive,'' said Given. ''It's mostly American stuff but there are Japanese shells as well.''

He said Australian navy bomb disposal units ran out of explosives while blasting at one site and had to use the World War II land mines they found buried in mud to detonate the rest.

Given said it would take ''many, many months'' to clear the flatlands outside Honiara of unexploded bombs, but the Australian demolition teams could only stay in the Solomons until June 19.

The Solomon Islands, a former British protectorate, became independent in 1978. During World War II the 900-mile-long Pacific island chain was occupied by Japanese forces. They were eventually routed by U.S. Marines after heavy land and sea battles.

The archipelago lies 1,300 miles northeast of Australia.