About three-quarters of the way there, one of the surf skis took on too much water in the choppy conditions and sank, he said.
Both men managed to clamber on to one surf ski and made their way to the western side of Mokoia Island.
However, Grouby said it was a rocky and steep part of the island and the men suffered cuts and grazes as they scrambled across the rocks to the shore, dragging their boat.
They then waited five hours up a tree - the best vantage point to wave for help.
Grouby said the men's wives contacted police about 4pm.
Coastguard and a local tourist company checked the lake but couldn't find the men.
"We knew they must have been on Mokoia Island. We have done this rescue several times before ... Ngongotahā is quite sheltered from the wind. When you look out you think it's a good idea to travel to that island and back but don't realise it will blow up and get rough."
He said the men were relieved when the Coastguard boat arrived and rescuers did well in 1m swells to get the boat close enough to get the two men down from the tree to safety.
"They were worried they'd have to spend the night in a tree ... It was a tragedy averted."
Grouby said while the men shouldn't have travelled without a means of communication, they did the right thing by staying with the bright yellow boat and waiting to get help.
Other callouts on the lakes at the weekend included a boat that broke down on Lake Rotoma at midday on Sunday and a catamaran that tipped over on Lake Tarawera mid-afternoon on Sunday, which ended up being rescued by local boaties.
Meanwhile, other visitors to Rotorua enjoyed brilliant conditions while attending the A&P show on Saturday and sporting events including the Ecomist Blue Lake Multisport Festival at Tikitapu (Blue Lake) and the Northern Region Junior IPS Touch Tournament at Ray Boord Park.
The thousands of participants and visitors meant affordable accommodation outlets were fully booked and takeaways and restaurants were chocker.
Factory Smokehouse & Grill owner/manager Jason Monahan said they had done well out of the touch tournament and their gluten-free menu meant they had attracted additional customers.
"We've done well out of the coeliac tourists and the touch tournament. But this summer has been different. Because it's been so hot and muggy, people aren't eating in that key time of 6.30 and 7.30pm. We get families nice and early, then we get a flat patch and then we get hit later on when it cools down."
Destination Rotorua's executive manager of visitor services Graham Brownrigg, who also manages the iSite, said Rotorua had more visitors this long weekend than the same time last year, totalling about 6700 through the Rotorua iSite over the three days. Saturday's numbers were similar to last year but Sunday's were up about 15 per cent.
He said the numbers were also up about 30 per cent on Saturday and Sunday at the Redwoods iSite over both days, proving the forest location was a popular alternative to the lakes.
