It will be the first chance for the members to see the new leadership team in action and judge whether they have a chance of leading National back into government in 2020.
Although National's party polling has held up around 45 and 46 per cent, Bridges is still polling low numbers as preferred Prime Minister, especially compared to Prime Minister Jacinda Ardern.
Key's own rankings were comparatively high from the moment he took over in 2006 and peaked at around 70 per cent in some polls when he was Prime Minister, but he said it was primarily a name recognition thing.
"Of course there will always be a focus on his personal numbers, but preferred Prime Minister is, in many respects, a name recognition issue. The incumbency gives you an enormous amount of benefit in that regard.
"Personally, I don't have any concerns at all about Simon's current personal numbers. I think the party numbers are ultimately what really matters and the other numbers will naturally track up over time."
He said Bridges was "growing" but it was harder to get profile in a non-election near, up against a new, young and popular Prime Minister.
However, he said the party numbers remained high and Bridges numbers would grow over time.
"It's a very difficult thing to maintain those numbers and what it shows you, I think is that the 44 per cent of New Zealanders have not wavered one iota since the last election. It is a tremendous effort to have held numbers at these levels."
Key said the job ahead of Bridges of toppling a Government after one term was a daunting one and had only happened twice in New Zealand's history, both times in unusual circumstances.
"I think everybody understands the enormity of the challenge, which is trying to defeat a one-term Government is not easy."
Key himself knows the difficulty of beating the odds - his aim had been to secure National a fourth term in Government although he stepped down and left the job of trying to do that to English.
He has kept a low-profile politically since stepping down, but said he was on hand to offer counsel where needed and was always ready to offer support.
Key said the party's supporters would be looking for signals on policies that were consistent with what the party's base believed in.
He said those voters were fundamentally economically driven, as well as focused on core issues such as law and order, health and education.
Key said the best chance for an uplift for National was if there was a downturn in the economy. At the moment, the fundamentals of the economy remained similar to when National was in Government.
"But if you see a real deterioration in the real economy - not just business confidence, but in what people feel and see - then I think you could get some very serious questions asked about the Government."
Those charged with capitalising on that will be finance spokeswoman Amy Adams and Bridges.
Adams will deliver her speech today, and is planning to focus on the impact National believes the Labour-NZ First government is having on the economy.
She is expected to point to changes in GDP growth, and has calculated that every percentage point drop in GDP growth equates to up to $800 million less in tax a year.
Bridges will deliver a keynote speech tomorrow and is promising change but not too much.
Bridges said it was too early to make a call on issues such as tax cuts although National was supportive of leaving more money in people's pockets.
"If we have the privilege of being Government, we don't yet know what we would be inheriting.
I think we are somewhat horrified really at what they are doing, where we see they've had a lot of money and their tax take on the current tax settings is up, but on top of that they've decided to impose a bunch of other ones - the most significant being the fuel taxes.
We will want to be showing some contrast to that. Exactly how that looks, obviously we can't say right now."
Housing Minister Judith Collins will also speak and said she intends to focus on what reforms are needed to the Resource Management Act to boost housing and urban planning.
