A public meeting about a recent decision to drop trains as an option for quick and congestion-free transport to the airport is expected to attract hundreds.
But Auckland Transport says it won't be attending because it would interfere with its "neutral position" on the option.
Campaign for Better Transport is holding the meeting on Tuesday evening to discuss the opposition to the decision.
In June, the boards of Auckland Transport and the New Zealand Transport Agency voted to eliminate the $2.6 billion commuter rail network option in favour of a $1.3b light rail or a bus option.
But the spokesman for the advocacy group, Cameron Pitches, said they believed the report the boards based their decision on was flawed.