Australia said on Friday it would pump an additional $3.2 billion into building the country’s nuclear-powered submarines, part of a controversial defence pact with the United States and Britain.

Under the 2021 AUKUS deal, Australia’s most expensive defence project, the country will receive three nuclear-powered submarines from the United States.

Australia and Britain will then build a new class of submarine in the 2040s.

But the deal has faced scrutiny over whether slow submarine production rates in the United States and Britain will leave Australia without the vessels it needs.

Defence Minister Richard Marles said the additional investment of Aus$4.6 billion (US$3.2 billion) at a shipyard in Osborne in South Australia “demonstrates that AUKUS is on track and happening now”.

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“This investment is another important step in building the sovereign capability Australia will need to build, operate, maintain and sustain our future submarine fleet,” he said

AUKUS is expected to cost Canberra as much as US$250 billion over the next 30 years.

Under the deal, Australia had been expecting to receive two used submarines and one new one, but it was announced in May that all three will now be in-service vessels from the US Navy stock.

Australia defended the new arrangement as more “cost-effective”.