Prime Minister Jacinda Ardern will have her audience with the Queen in London tonight as the Commonwealth Heads of Government meeting kicks off at Buckingham Palace.

Ardern will spend most of today at Buckingham Palace - from the "family photo" in the ballroom, the official opening of CHOGM and a reception tonight for the new leaders in the Commonwealth, and a private audience with the Queen - her first meeting.

Ardern, who lost her cat Paddles last year, may be able to offer a sympathetic ear to the Queen whose last corgi - Willow - died on Sunday.

It follows a hectic day in London overnight in which Ardern was to meet with four of the Five Eyes leaders - British PM Theresa May, Canada's Justin Trudeau and Australia's Malcolm Turnbull. The meeting was convened by May to discuss an alleged cyber-espionage attack on governments, infrastructure providers and businesses, which Russia has been blamed for.

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Ardern also spoke to youth at City Hall with Trudeau and London Mayor Sadiq Khan, had an audience with Prince Charles and Camilla at Clarence House, and met Fiji's Prime Minister Frank Bainimarama for the first time before heading to Downing St to meet May.

The day ended with a welcome reception hosted by May for more than 50 leaders in London for the CHOGM summit.

There is a poignant symbolism about it being in London this year - it is expected to be the Queen's last CHOGM, an event she had always attended apart from in 2013 when she instead sent Prince Charles to Sri Lanka.

It is also perfect timing for May who is set to seek support in the case against Russia over the use of a nerve agent against former Russian spy Sergei Skripal and his daughter Yulia. While some have already expressed support, including New Zealand, others are more reluctant, such as India.

May will also be looking for close friends as Britain prepares to leave the EU - while New Zealand is already in the line, other countries are likely to want to join the queue for free trade agreements.

London was dressed to impress - the Mall and Buckingham Palace were bedecked with the flags of the Commonwealth and even the weather was working to Her Majesty's orders - sun was expected all week after a long winter.

Between the pomp, the feasts and the royals the leaders will grapple with the issues that beset the group.

There will be a big turnout - all but two of the 53 countries in the Commonwealth are represented by their leaders - including India's Narendra Modi, who has skipped the last few CHOGMs.

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Tomorrow the leaders will go into a retreat at Windsor Castle but on the sidelines until then major international issues will also be playing out - most of which do not involved a Commonwealth nation but rather the United States and Russia.

The hot topics in leaders' conversations are expected to address attempts to get UN action on the use of chemical weapons in Syria, and May's call for Russia to be condemned over the nerve agent case.

The "trade wars" threatened by US President Donald Trump's new tariffs could also re-ignite debate about a "Commonwealth Trade Area", something NZ First leader Winston Peters supports but which would be complicated and tricky given the diversity of the countries involved.

ABOUT CHOGM:

• 53 countries, of which 16 have the Queen as head of state.
• Meets every two years
• London took over hosting after the 2017 cyclone in Vanuatu.
•This year's leaders retreat (overnight Friday NZT) will be held at Windsor Castle.

What will they be talking about?

RUSSIA:

Russia is not in the Commonwealth but British PM Theresa May has called for support from partners over the use of a Russian nerve agent on former Russian spy Sergei Skripal. There is also the ongoing fallout over chemical weapons use in Syria and alleged cyber attacks on government agencies and private businesses. Consensus among the leaders is unlikely - leaders like India's Narendra Modi are seen as more sympathetic to Russia.

CLIMATE CHANGE: Leaders will be boasting about the steps taken so far to meet Paris Agreement accords, including Ardern who can point to her decision not to allow future oil and gas exploration permits.

TRADE: Many leaders are concerned about the impact of increased protectionism from Trump. Brexit means many countries will be sorting out how they deal with Britain in future in terms of trade. New Zealand was quick - we are already in the top three expected to get free trade agreements with Britain. But Brexit has sparked support of a "Commonwealth-wide" trade deal, something many believe unlikely because of the diversity of economies in the Commonwealth.

THE QUEEN: Leaders will decide whether the Queen's successor as monarch will also become head of the Commonwealth, or whether the role should instead be elected, rotated around the countries, or disposed of altogether. It is a symbolic role, but the Queen has been quietly lobbying for 2013 to ensure her successor keeps it.

ZIMBABWE: Leaders will consider whether to allow Zimbabwe to re-enter the Commonwealth.

ELECTIONS: The leaders are expected to sign off on revised guidelines for election monitoring, a project former NZ Governor General Sir Anand Satynand has been involved in.