"We are building up the momentum and the guys have started believing in themselves," Lara said after his team's crushing six-wicket victory over second-ranked South Africa in the semifinal here on Thursday.

The West Indies will clash with world champions Australia at Mumbai on Sunday.

Left-handed opener Chris Gayle slammed an unbeaten 133 as the West Indies surpassed South Africa's challenging total of 258-8 with six overs to spare in the day-night match.

"We just have to keep the momentum going and analyse each game ... win or lose, just go back and think about it. I think it is going to be exciting times ahead for us, especially in one-dayers," said Lara.

"It's nice to be playing the world champions in the final. It says a lot about our effort... getting past South Africa, and beating India and Australia in group matches. So, we are looking forward to the challenge."

The West Indies began the tournament on a sensational note when they defeated Australia in their opening group match at Mumbai, but Lara said the final at the same venue would be different.

"Australia have gained some momentum since they lost to us. They are very confident of their chances. We have to realise that we are going to play a completely different opposition to the one we played against," he said.

"That time Australia were playing their first match and we had a sort of an advantage. It's a challenge but we will back ourselves."

Lara said he was confident his side would deliver in the final because they had been playing good cricket recently. His team beat India at home and then qualified for a triangular series final in Malaysia before losing to Australia.

"I won't say it's an unfinished business against Australia. I just feel it's nice to play them in the final again," said Lara.

"Our one-day game has always been pretty good. It just required a little bit of fine-tuning. We did that in Malaysia. We were forced to play the qualifying round here and that in itself gave us a little bit of competitive cricket."

The West Indies beat Bangladesh and Zimbabwe in the qualifying round to make it to the main draw of the ongoing tournament.

Lara said his team owed the victory against South Africa to left-handed openers Gayle and Shivnarine Chanderpaul (57), who retired hurt due to cramps but not before adding 154 for the first wicket.

"The way Gayle and Chanderpaul started, they transferred the pressure on South Africa. It was a good effort and everybody must be commended," said Lara.

"They came out ready to put pressure on Shaun Pollock, Makhaya Ntini and Andre Nel. The fact that in 15 overs or so, we were almost close to 100 put them on the backfoot. They just had to play catch-up cricket after that."

The West Indies were never under pressure after the Gayle-Chanderpaul stand even after losing four wickets, including that of Lara, in the space of 48 runs.

"We always thought the target of 259 was gettable and the guys talked about it," he said.

"They thought the pitch looked very good and it would stay that way for the entire 100 overs. So, it was a good toss to lose. It's a situation from where we just have to move on."