Chief Minister Katy Gallagher proposed the joint bid to NSW Premier Barry O'Farrell last month after a strategic review of the ACT's capacity to host a major international event which found its hotel and convention centre facilities would not cope with such high-level demand.

Instead, the ACT will back NSW to host the G20 Leaders' Summit if the ACT secures a number of the major side events in the lead-up to the main event.

Senator Humphries said the ACT should be equipped to win the entire summit and was copping out in joining with NSW.

He singled out ACT Labor senator Kate Lundy as a hypocrite for supporting the joint bid when she has vehemently argued for Canberra to be the sole host for the Commonwealth Heads of Government Meeting in 2000.

Former prime minister John Howard moved CHOGM to Queensland after he was advised that the ACT was not able to meet the logistical demands of the meeting. Senator Lundy argued at the time that he was Canberra bashing, and she said the ACT was ''fully prepared to host CHOGM''.

Senator Humphries asked her to explain how Canberra could be ready to host 55 heads of government through CHOGM but not 20 leaders at the G20.

Senator Lundy said yesterday she supported Ms Gallagher's decision because the joint bid was ''about seeking a good outcome for Canberra and is based on advice from the independent Canberra Convention Bureau. Canberra is a fantastic destination and I support the bid to showcase our capital on the international stage''.

Senator Humphries said the ACT had continually missed opportunities to host international events and Labor had been in power locally for 10 years since the CHOGM knockback but had failed to provide better infrastructure to allow the ACT to fulfill its potential as a national capital host.

''If Labor knows it is not logistically possible for Canberra to host, then it should be incumbent on them to change the facilities. I had assumed by their lack of action, and by Senator Lundy's earlier arguments, that Labor was confident Canberra had the capacity to host something like CHOGM or the G20. Now we see they are not.''

According to the strategic report on Canberra's hosting capacities - commissioned by the ACT Government in November and conducted by the Canberra Convention Bureau - the ACT has only 5250 hotel rooms available when the G20 requires between 7000 and 10,000.

Senator Humphries said the ACT could bring more hotel rooms into operation before the 2014 G20 deadline and even suggested some delegates could stay in Queanbeyan or Sydney. ''Admittedly it might be tight but you can always make arrangements,'' he said.

With the report also noting the Convention Centre - with a capacity of 2000 - was not up to G20 standard, Senator Humphries said Parliament House could host the event.