Laws to ensure fair political representation are threatened in three of the Bay of Plenty Regional Council's four big electoral wards.

Tauranga Mayor Greg Brownless has challenged whether the region's electors would be fairly represented on the council after the 2019 election.

The region has proposed keeping the current representation formula that gave Tauranga five councillors, Western Bay, Rotorua and Eastern Bay two councillors each, and the three Maori wards wrapping up the 14-member council with one member each.

The council had already conceded that it will be outside the law for representation ratios for the Eastern Bay and Rotorua.

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And Brownless said that Tauranga electors would be almost immediately under-represented if the status quo was retained.

Rotorua and the Eastern Bay wards fell outside the Local Electoral Act's 10 per cent plus or minus ratio for the Bay's average representation of 22,127 people per councillor.

Eastern Bay would be over-represented by being 25 per cent below the average population per councillor, and Rotorua under-represented by being 12 per cent above the average.

Brownless was due to deliver his submission to tomorrow's meeting of the regional council.

He said the regional council's plus or minus 10 per cent range equated to a representation of 19,914 to 24,340 people per councillor.

The council had calculated Tauranga's representation on a population of 117,700 - 48 per cent of the region's total population.

Brownless said that based on this population, Tauranga was already at 23,540 people per councillor - close to the upper limit allowed by the Act.

He then quoted Statistics New Zealand's last official population estimate of 131,500 at June 30, 2017.

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''It would seem short-sighted to put in place representation arrangements that will almost immediately result in the 10 per cent greater or small ratio being exceeded.''

Brownless will ask the region to either increase the number of members for Tauranga or carry out its next review well before the maximum allowable six years.

Tauranga's population was predicted to reach nearly 146,000 in six years - 28,000 more than last year's figure.

A report to tomorrow's meeting justified the representation for Rotorua and the Eastern Bay by quoting the Local Government Commission. It said the commission had previously noted that the territorial boundaries were a good reflection of natural communities of interest.

''Little change has occurred to alter these distinct and natural communities of interest.''

It said the Eastern Bay of Plenty was formed by amalgamating the Kawerau, Whakatane and Opotiki districts into one constituency (ward).

''Current boundaries cannot be realigned in a way that will not alienate communities of interest or create unnatural geographical boundaries,'' the report said.

The council discussed representation options at a workshop on April 23. Five scenarios were considered including altering the boundary of the Eastern Bay to try and better meet fair representation requirements.

The Maori wards were within plus or minus 5 per cent of their average political representation of one councillor per 18,900 people.

Deviations from average 22,127 representation per councillor in the general wards
Tauranga (5 councillors/117,000 popn): +6.4 per cent
Rotorua (2 councillors/49,700 popn): +12.3 per cent
Western Bay (2 councillors/43,000 popn): -2.9 per cent
Eastern Bay (2 councillors/33,000 popn): -25.4 per cent
Source: Bay of Plenty Regional Council