The head of the foundation given a $444m grant to protect the Great Barrier Reef said it is uniquely placed to fundraise and deliver environmental projects, in response to criticism from Labor about the grant.

Anna Marsden, the managing director of the Great Barrier Reef Foundation, conceded the grant came as a “complete surprise” when first proposed by Malcolm Turnbull and the environment minister, Josh Frydenberg, but rejected Labor claims about the foundation’s fundraising record and plans for the money.

Labor has used a Senate inquiry to campaign against the grant, questioning why environment department officials were not present at the initial meeting on 9 April when the money was offered and, on Tuesday, calling for the foundation to voluntarily refund the entire grant.

Asked about the fact the foundation did not apply or complete a competitive tender for the grant, Marsden told ABC’s 7.30 it had come as a surprise.

But she said the reef had lost half its coral after back-to-back years of coral bleaching and cyclone Debbie so the reaction was to “breath a sigh of relief” that they had the means to make a difference.

Asked why the foundation received the grant, Marsden said it was the largest reef charity and the government wanted to encourage co-investment in the reef.

She cited the foundation’s record delivering public-private partnerships and said it had raised a total of $90m over 18 years for the reef, rejecting Labor’s suggestions the total may be closer to $50m.

Marsden said that 80 cents in every dollar raised was spent on reef projects, which would rise to 90 cents a dollar for the federal government grant.

Marsden identified climate change as the biggest threat to the reef, adding that efforts to protect it should “pull both levers: resilience and mitigation”. She said Australia “could probably be doing more” to meet its Paris targets and prevent climate change.

Marsden disclosed that the foundation’s business partners included BHP, Google, Lendlease, Qantas, and Rio Tinto, but said they had “no influence” on project selection or design.

On Monday Turnbull defended the government’s “very thorough process” before the grant, explaining that both the expenditure review committee and cabinet had approved it before the money was offered.

Turnbull explained the government had not put the grant out to a competitive tender because the foundation was an “outstanding” organisation and they “were clearly the best team to do it”.

“What the Labor party is doing now is – they are embarrassed that they did not put serious funding into the reef,” he said.

Frydenberg has also dismissed concerns about the grant, arguing there was “a lot of transparency” because the agreement with the foundation was public and the audit office will be able to track all its spending.

Labor’s probity concerns include the lack of competitive tender and the concern it represents a “privatisation” of functions of the environment department.