A LAST-MINUTE increase in crowds at Adelaide Oval for upcoming AFL finals has been approved with 20,000 fans to be allowed.

Port Adelaide is due to play Geelong in the second qualifying final at the oval on Friday night, with Melbourne playing the Brisbane Lions in the first qualifying final on Saturday.

In recent weeks, crowd numbers were capped at 15,000 due to COVID-19 restrictions but Premier Steven Marshall said everyone wanted to see more fans allowed in.

"Clearly, we would like to see the capacity increase if it is safe to do so," he said on Thursday before the increase was announced.

"But our primary responsibility is to make sure that we keep South Australia free from this Delta variant. We want to keep it at bay, no South Australian wants to see us go into lockdown.

"But if we can safely have more people at Adelaide Oval, we'd be keen to do that."

Mr Marshall said Chief Public Health Officer Nicola Spurrier was looking at the latest information and it was hoped some advice was forthcoming.

With SA Health approving the 5000-seat increase, the extra tickets went on sale on Thursday afternoon.

In a statement, Port Adelaide said it was thrilled more of its members would have the opportunity to attend the match on Friday.

But it said with the stadium well below capacity, many would still miss out.

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The premier said it was disappointing that Adelaide had not been chosen as the backup venue to host the AFL grand final, after putting forward what it thought was a competitive proposal to have up to 30,000 fans.

"We put our best foot forward. We gave it a go. Unfortunately, we weren't successful," he said.

"We could not match the 60,000-plus crowds that they could have over in Western Australia."

As a Port Adelaide fan, the premier said his focus was now on the Power beating Geelong on Friday and then going on to win the flag.

"Wouldn't it be good to see a Port Adelaide premiership? It doesn't matter where it is," Mr Marshall said.