AMSTERDAM, Nov 28 (Reuters) - Dutch insurer Achmea said on Thursday it had reached a deal with investment firm Sixth Street to form a new life insurance joint venture, aiming to profit from the overhaul of the Netherlands' large pension system.

Sixth Street will merge its insurer Lifetri with Achmea's life insurance and pensions portfolio and will pay Achmea 445 million euros ($469 million) in return for a 20% stake in the new company, Achmea said.

Achmea will get the remaining 80% of the shares in the joint venture, which will remain part of the Achmea group, the Dutch insurer said.

The new venture will aim to play a big part in the consolidation of the Dutch pension sector, as a result of the overhaul of the country's 1.7 trillion euros private pension industry, Europe's largest.

Achmea said it expects a total of 20 to 30 billion euros in funds will be available for buy-outs by insurers in the coming years, as pension funds grapple with the costs of the overhaul.

The new joint venture expects to be able to take up to 20% of those funds, the insurer said.

The joint venture will have around 2.1 million customers, with around 500,000 from Lifetri, and would be in the "top three" of life insurance companies in the Netherlands, Achmea said.

Its main rivals are insurer ASR and Apollo Global Management-backed Athora.

Achmea, an insurance cooperative backed by Dutch lender Rabobank, had said in February it was exploring options for its Pension & Life Insurance arm.

At the time, sources said the company had held early stage discussions about a potential sale of the operations, including with rivals NN Group and Athora.

($1 = 0.9489 euros)

(Reporting by Bart Meijer Editing by Mark Potter)

((Bart.Meijer@thomsonreuters.com;))