"They had to abandon the vehicle and that's when it all happened," Mr Simpson told the Weekend Herald.
"One of the guys grabbed the little girl and Sandra at the same time, but he lost his grip on the little girl because the river was running that fast, he was up to his chest in water."
Mrs Saunders survived but little Emily is still missing. The search for her has been suspended until Monday due to bad weather.
Searchers found the body of Mr Petrie just after 2.30pm yesterday.
Mr Simpson said he was devastated by the loss of a little girl who was "like a grand-daughter".
"We treated her like our grand-daughter, that's why it's knocking us around a bit," he said.
"I've got grand-kids around the same age."
Mr Simpson said he was more than just Mr and Mrs Saunders' employer.
"We're friends as well. They're some of the best workers we've had working for us."
Mr Simpson said Emily's parents were "blown away".
"We all are, everyone's very upset."
They appreciated the support from the community, who have rallied around them and the Petrie family.
"We've had marvellous support from the district, its only a small town about 500 people and they're bringing out food and cooking for them and all sorts of things," Mr Simpson said.
He was well known throughout the area and locals were keen to pay tribute.
Federated Farmers West Coast provincial president Katie Milne said Mr Petrie was an "awesome fella", an "absolute stalwart".
She said he had been involved with pest control work and in helping with TB eradication.
She said the incident was "awful" and something the community didn't need.
Hari Hari resident Mary Malloy, who had known Mr Petrie for 30 years, said if he could have helped anyone on that river he would have done so.
She said he had been a successful triathlete and had once been her husband's biggest competition.
Mrs Molloy said the incident was horrible for Mr Petrie's family.
West Coast police area commander Inspector Mel Aitken said inclement weather conditions had limited the search. Conditions were set to worsen over the weekend and teams would resume looking for Emily on Monday if they could.
"Our thoughts are with the families on this tragic occasion," he said.
"This is a particularly hard time for everyone involved."
When emergency services arrived at the river on Thursday the vehicle was partly submerged. But within 10 minutes it was fully under water.
"You couldn't see anything," said Hari Hari fire chief John McIntosh.