"When an extraordinary vacancy occurs more than 12 months before the next triennial election, then the vacancy must be filled by an election. It's a legislative requirement under section 117 Local Electoral Act 2001," Mrs Moosman told the Chronicle.
That legal requirement means the council cannot appoint the highest-polling loser from last October's elections - in this case, radio host Jason Granville.
No decision on when the by-election will be held can be made until council formally receives Mr Laws' resignation. The by-election must be held not later than 82 days after the date the electoral officer receives notice of the vacancy.
Mrs Moosman said a by-election would be conducted under the Local Electoral Act 2001 and the Local Electoral Regulations Act 2001, and would follow exactly the same process and timelines as a triennial election.
In last year's elections, nominations were open for a month and postal voting papers went out about five weeks after that closing date. Voting was open for about three weeks.
There will not be a by-election to fill a vacancy on the district health board. The spot would be filled by an appointment made by the Minister of Health.
In the October elections Mr Laws was 2650 votes behind Ms Main in the mayoralty race and polled sixth highest in the council election. He finished only a few votes shy of the top-polling candidate in the health board elections.
Meanwhile, councillors Ray Stevens and Jack Bullock want council to press for change to the legislation.
Mr Stevens said a by-election would cost the community at least $40,000 and he believed it was time council discussed the issue and promoted change.
"Cr Jack and I believe the mood of the community is with us and we should make immediate endeavours to progress a change in legislation," he said.
"A by-election is an unnecessary compliance cost on the community at this time that serves no purpose."
Mr Stevens said council should be taking the matter up with Local Government New Zealand and the Government.
"Let's not procrastinate. I raised this four years ago and again last year, so let's move, if not for this time but for next time ..."
Meanwhile, a Facebook page has been set up to promote Mr Granville being given the seat to avoid the by-election.
Mr Granville missed out on a council seat in October's elections by 81 votes.