Liberal Democrat Aled Roberts asked why Betsi Cadwaladr University Health Board had spent so much filling doctors' shifts over the last eighteen months.
The board is currently being supported by government officials as it tries to improve its management.
The Welsh government said spending on agency staff would be tackled.
A new team of managers is trying to turn the board's fortunes around after a critical watchdog report in 2013 led to the resignations of several senior figures.
Mr Roberts told the assembly's Public Accounts Committee on Tuesday that Betsi Cadwaladr had spent £12.5m on locum doctors in 2013/14 and £8.3m during the first five months of 2014/15, and that expenditure on agency nurses had also increased substantially.
He asked government officials: "Have any of you actually been on wards and discussed how chaotic the whole situation is, where basically they decide at 5 o'clock in the evening how many nurses they are going to phone up for?"
Committee chairman Darren Millar added: "There is significant potential here that the committee sees for cost reduction whilst maintaining levels of service and the staff complements that the board needs.
"It is a concern to us that we are seeing rising agency costs at a time when we would anticipate that this is one of the areas that needs to be focused on in terms of cost reduction."
Dr Andrew Goodall, director general of health and social services at the Welsh government, replied: "This is one of the areas that we will be focusing on with the intervention.
"We do have an expectation that locum and agency spend is reduced."
A £20m bill for locum doctors in north Wales raises "serious questions" about "chaotic" workforce planning by the local health board, an AM claims.