The show's interior designers Shaynna Blaze and Darren Palmer predict the apartment of the couple, who turned their wedding into a season finale, will bring the highest sales price.

Blaze says the couple have "the biggest advantage in every way" coming into the auction. The apartment has city views and a desirable aspect and the couple have won plaudits for their design. ​

Palmer also went with Max and Karstan: "If you can pick the winner of this auction you deserve all the money that you get because the odds have to be all over the place."

Sources close to the selling agents also suggest the buyers with the deepest pockets are circling Max and Karstan's generously sized, north-facing apartment.

Neale Whitaker, the former editor of Belle magazine, tips Michael and Carlene, who have a southerly aspect but the second largest terrace on The Block.

In series nine, five couples were given the task of converting a Prahran office block in east Melbourne into five luxury three-bedroom apartments.

The series underdogs are Sydney couple Chris and Jenna whose living and dining room renovation was compared to a display home.

They have the largest apartment to make up and are the only team to have a rooftop terrace. Last year's winners, Steve O'Donnell and Chantelle Ford, have named them as the couple to beat.

The three judges have had a patchy record picking winners, naming Alisa and Lysandra, who  agreed with Blaze and Palmer, as winners of the The Block All Stars.

But they failed to pick last year's winners, Ford and O'Donnell, who won series eight with the biggest sale in the show's history.

Unofficial expectations of up to $2 million per apartment have been thrown around for months, but on the eve of the auctions, Channel Nine would not speculate on numbers.

​Only Nine's camera​s, media and bidders who registered with each team's agent were cleared to attend​ the confidential auctions on Saturday.​

A crowd of about 40 fans, onlookers and prospective bidders mingled outside the High Street complex as the nervous contestants filmed their final minutes on camera and prepared for their homes to go under the hammer.

They are all under strict instructions not to reveal the results before the grand finale goes to air Sunday.

​Such is the level of secrecy ​imposed by Nine that the contestant​s'​ real estate agents ​were unaware of the reserve prices until the eleventh hour and have been guiding prospective buyers without that basic knowledge.

Fairfax understands many potential buyers have registered to bid for all five apartments, further complicating the agents' guessing game.

Darren and Dee's agent, Russell Cambridge of Biggin & Scott, said the order of the auctions will likely determine the winner.

"There are many variables that don't come in to play in real life," he said. "At the end of the day, it is still a game and a reality show." ​

The Block started in Bondi 11 years ago, with renovations following  in Manly and Vaucluse, before the production shifted to more affordable Melbourne.

Contestants collect the money paid above the reserve and the winning team also receive an extra $100,000. Dingle Partners' Anton Wongtrakun, who is selling Chris and Jenna's apartment, said if sales are lucrative, it will prove to developers there is a cashed-up market for more spacious, luxury apartments.

"If there is proven depth in the market it might encourage developers to consider more substantial properties," he said. "In inner-city urban areas, developers have been reluctant to scale up to this level."

Greville Pabst, CEO of property valuers and advisers WBP Property, has walked through the Glasshouse. Due to workmanship, furnishings and publicity, he expects the apartments to exceed market value and attract above $2 million.

But he does not expect big sale prices to be a dangling carrot for developers. "These types of properties offer limited profitability for developers, particularly given the high standard to which the properties in the current season are finished," he said.

The man who built The Block empire, co-creator and executive producer Julian Cress, said he has sat with contestants during previous auctions who have won up to $700,000 and with those whose apartments didn't sell.

Mr Cress takes seriously the business of building and selling prestige real estate.

"We are about creating boutique developments because that sits within our format," he said. "If this property was tackled by a developer, they would put in 50 apartments, but we put in six.

"Not only do we create properties of a level of comfort and luxury, but we also price them reasonably. When our contestants have made money it is because the reserve prices were realistic and gave the contestants a chance."

With Linda Morris